First of all, it is hard to put specific guidelines that fit on every situation. It is just impossible. As for you, it seems like you are WAY to loose. If you can become a chip leader at the beginning of the tournament, though it is great, you have to be playing very very loose (unless you are getting like pocket aces every single hand). You need to tighten up and play a whole lot less starting hands. As for what hands to play later in the tourney, you should just play the exact same hands, which are only good ones. Don't even bother playing bad hands at the beginning of the tourney, you are just wasting chips. To be honest, 100/200 is not really mid game. On PS, 100/200 are the blinds only after 1 hour 15 minutes. I would consider mid game for a freeroll on PS over 2 hours into it. But whatever, that doesnt really matter. As for bluffing, its a tricky thing. We have all had time where we raise big with AKs, have a few callers, and then completely miss the flop. It sucks. It really does, but we can't do anything about it. Overcards will not always hit. Whether you fold or bluff depends on what your opponents are doing and what your reads on them are. If they lead out with strong bettings, you should probably fold. If they check to you, throw out a bet, maybe about the size you raised preflop. If someone raises you, you should probably get out of the hand. If they call, you are in a tough situation, because you can't always tell what they have. They might have something very good and will never fold it or they might have a low pair and will fold to a big bet. You just have to use your feelings and intuition. I would suggest not trying to bluff a whole lot. Save it for certain hands every once in a while. If players see that you are always betting at the pot and stealing it, they will have a tendeny to start calling you. I hope this has given you some sort of help. I think the main thing I want you to get out of this is to tighten up and stop playing a lot of hands, especially at the beginning of the tourney.
Read this over, I definitely think it will help you:
http://www.texasholdemforums.com/showpost.php?p=13988 (Cowboy's Preflop Guide For Beginners)
Also, look over this page and read as many different guides as you want. They are all great and very informative:
http://www.texasholdemforums.com/member_guides.php
Problems mid game 
