As lame as it sounds, Microsoft Flight Sim in multiplayer cannot be beaten for learning the basics of formation flying
Unfortunately, this isn't true.
The most important aspect of formation flying is NOT the physical act of staying in position on the wing. The most important part is knowing how to handle unusual and emergency situations safely. MikeD talked about several of these in his post in the other current thread on this subject.
This is something that can only be taught and practiced by someone who knows and understands what they're doing.
It might be smart for you to ride along on a formation flight at a FAST weekend -- it might surprise you what all goes in to learning how to safely fly formation as well as execute a flight.
It is VERY dangerous to do this when you don't know what you don't know. Remember, with airplanes flying close there is simply zero room for error. If one of the two of you makes the wrong decision about what to do, then you will both probably die as a result of the collision between the two of you. Unfortunately, it almost doesn't matter how good YOU are...it only matters how good the other guy is, because if he screws up you are probably going right with him.
One thing I've learned through my warbird flying is that even FAST trained/certified have significantly varying levels of training and proficiency. It is surprising that guys with FAST cards don't have more collisions than they do....hell, it's surprising enough that military trained guys don't have more midairs than they do! Some of the stuff I've seen out of advanced students is amazing -- I used to joke that flying with students in formation was hundreds of times more scary for me than nearly getting shot down by an Iraqi SAM because with students I knew that they might randomly try and kill me at any time during a formation sortie.
The bottom line here is that formation flying doesn't take any amazing skill or smarts to do safely -- it just includes many aspects that aren't inherent to any pilot who wants to do it. I encourage people to do it, because it's great fun and develops a new skill in aviators...but YOU SHOULD go get some formal instruction in it, and only fly with wingmen who have received similar instruction.