Below are the two most common methods of surrounding the Flag with Bombs. The first one is in the back row, up against one of the edges of the board. This method uses just two Bombs. The second method is anywhere else in the back row and requires three Bombs.

Killer Pack - I group four pieces together, two on the front line and two behind, to form a strong unit.  For example:

Scout     Sergeant
Miner     Colonel

These four pieces work together to dent the opponent's line.  Variations include substituting a Sergeant with a Major and the Colonel with the General if the Spy is nearby.

Additionally, I try to occupy territory between and beside the lakes.  This allows me room to swing pieces back and forth on my side of the lakes.

Along with the idea of preserving mobility, I almost always start my Marshal and Spy in the middle of the board.  This allows me to counter a big threat on either edge.  Since the Marshal is so important, I don't think he can adequately be balanced by the General in a split forces strategy as you suggest.

When possible, keep the Spy behind the lake areas to prevent him from being nabbed by curious enemy Scouts.

Set up Scouts in the columns unobstructed by lakes and Bombs to maximize their movement and usefulness.

If your opponent is fleeing from one of your powerful pieces, try to move your piece in a position to threaten multiple pieces.  Your opponent will only be able to save one of those, and you can capture the other.  Conversely, avoid this if you are fleeing a powerful piece yourself, unless you have a counterattack planned.

I have a set up that really DOES have a logical advantage and it can win you the game even if you fall behind. Also, it can buy you some time to get an attack going when you are backed up.  I call it the Double Bomb Defense.  You have enough Bombs to make two complete rings around your Flag.  You have to put the Flag in the corner with two Bombs around it.  Then you put Sergeants or Lieutenants around them.  Next, you put a complete diagonal of Bombs defending the entire corner.   Your opponent will need at least two Miners to get through, and in most cases he will need three or even four.  You will surely capture one or two Miners before they get up to your Bombs, so I am pretty confident I will not lose any games when I use this defense.

Go for you opponent's Miners with a vengeance.

Move a lot of different pieces.  You opponent will not forget what you have if you only move one or two pieces at a time.  Make sure you don't have single men running around; they are way too easy to keep track of. Have multiple attacks going and move lots of pieces up from the rear.

 Let's face it, the Lieutenants are usually quite expendable and useless in a game. They don't have any cool maneuverability, like the Scouts, or any Bomb-defusing ability, like the Miners, and yet their rank is too low to really take anyone out in an attack. Well, I have devised a way to get absolutely the most use out of them as possible. Put a line of them between your Flag and a shield of Bombs that protect your Flag. The reason for this is three-fold: First, if a Miner comes down to break through your wall of Bombs, you are right there with a Lieutenant to take him out. And what can be more insulting than killing a Miner with a Lieutenant! It's classic. Second, your opponent usually does not expect any sort of counter-attack, at this point. After all, he thinks he has won the game! But your Lieutenant is there to kill off the Miner, giving you time to get some better pieces over to assist. Third, you can use your more valuable pieces in other attacks instead of having them trapped behind Bombs for the entire game.

Send out a group of four men, one high-ranking officer and the rest just duds. Only attack with your high-ranking officer so your opponent thinks you have a whole group of high-ranking officers. This works well because not only will your opponent be intimidated by this "army" you have sent out, but they may think all of your high-ranking officers are attacking and underestimate your strength back on the home front. They may try to attack around the back, just to find out you are very well prepared for him/her.

Seal off the two sides of the board with Bombs so nobody can penetrate them. This ensures the main battles, at least at first, will take place in the center of the board. You can then just stack the center with all of your high-ranking officers and blast your opponent. By the time your opponent brings his/her Miners over to defuse the Bombs, the game will be all but over.

Send a Miner and your Marshal across the board as your main attack pieces and then let the Scouts jump clear across the board and dictate which pieces you should attack before you actually attack them.

 I put Sergeants and Lieutenant in the front rows.  They aren't very useful to begin with, so they might as well be used to "scout" the opponent's front lines without sacrificing an actual Scout.

Similar to Wade Oliver's tip, use two dummy Bomb setups, one in each corner.  Then place your Marshal, General and Flag in the middle somewhere.  Your opponent will probably concentrate his/her attacks on the corners - where your Flag is not.

I myself would never set up my Flag this way. Remember, there are two ways to win at Stratego and capturing your opponent's Flag is only one of them. The other way, of course, is to prevent your opponent from moving by capturing all of his moveable men. This setup locks in all three of your Majors, one of your strongest officers. Unless your opponent uses a Miner to destroy one of the Bombs (except the upper-rightmost Bomb) these officers are effectively "trapped" for the entire game. If you and your opponent each lose your Marshal, General, and Colonels, his three Majors will reign supreme and will easily win the game for him. - Ed

 I believe some of these tips are quite good and very effective but greed often foils well thought of plans of defense. You must admit it still holds true - the best defense is a good offense.

You must continue to probe and try and confuse your opponent's memory of the locations of you key pieces, and learn when to trade down, power, and when to bait & wait.

Don't move too much too soon, since it often allows your opponent to visualize quite accurately your Bomb placement and most probable Flag location.

Make them waste their Miners whenever possible.

 Your Spy should be treated as an "XO" and never left more than a space from your General.  That way when/if your General is killed by the opposing Marshal, your are able to eliminate the opposing Marshal with your Spy on the next move.  I also would assign a Major or Captain to travel with this group, for Spy protection and extra flexibility.  This grouping can become obvious as you move them all together, so I usually designate my General as a more "defensive" piece, and my Marshal as more of an "offensive" one.

Decide early on if your strategy is to capture the enemy Flag or decimate his ranks.  If you choose the latter, move quickly to eliminate enemy Colonels and Majors.  If successful, follow up by sacrificing your own Marshal and General in exchange for those of your opponent.  This way you will have "command of the board." Immediately after having such command, designate one "invincible" officer to "Flag detail" and use another to seek out and destroy pieces/the Flag in enemy territory.  If your strategy is to capture the enemy Flag, play somewhat more defensively and let your opponent bring the battle to you.  It often becomes apparent where your opponent's Flag is by his movements.

Place Sergeants next to Bombs to quickly eradicate enemy Miners.

Scouts become more useful later in the game.  Note the Spy is nothing more than a slow-moving Scout after the enemy Marshal has been eliminated.

The Marshal and General are much more valuable in the middle of the board.  It's often a bad idea to position them too far to one side. 

 

This is the setup in the corner of my board:

Most of my opponents seem to like attacking the extreme left and right columns.  When a Miner gets down this far, 90% of the time they'll diffuse the leftmost bomb in the illustration above, rather than the center one.  I then kill the Miner with my Sergeant, and most opponents assume this was a typical Sergeant trap and totally ignore this side of the board (even if powerful pieces like the Marshal are with the Miner).  This can provide a valuable period of confusion as the opponent focuses on my other bombed-in Sergeant trap at the other end of the board.  This strategy usually allows me to not have to protect my Flag at all (which further adds to the illusion in my opponent's eyes), and I can deploy all my high powered pieces in the middle and on the opposite side of the Flag.