In a nutshell:

Raise bigger preflop, look for cheap (!) flops (but don't exaggerate, 43s is not a great hand if you see the flop 6-ways), play aggressively postflop with strong hands but be careful about "hand strengths". TPTK is not actually a strong hand 5-ways where players see the flop with all kind of hands. If you bet strong, you likely kick out all crap (that you had beaten anyways) and get milked with 2pair+.

Test the water, check, evaluate, stab (small) when nobody seems interested, play nuts to the max.

Hi

2 site recommendations and what I do:
- exploit the mistakes:
- population wants to see loads of flops and will play fit and fold.
- ie limp call with very wide range (can be as wide as 56o or k5o to aa.
- fit means calling any pair and any draw including gutshut on the flop.
- fit means raising anything as good as 2 pairs and maybe top pair any kicker on flop.
-on turn fit means top pair + maybe second pair if top pair came on turn.
- they don't re evaluate strength, or if they do, still act as if they haven't. Ie call kk to the end, call all in the 2 pairs if flush came etc.
- slight exagérations but looks a lot like that.
- so you need preflop:
Open and 3bet strong hands higher (ie more bb than the rest of table does)
You can call limps with low pairs and suited connectors but not anything, basically what you'd normally open with.
As aggressor, small cbet, at turn if heads-up you can bluff villain with biggish bet (often big in absolute terms, not necessarily percent of pot). If multiway at turn, you need strong hand. If you get to river, need strong hand too, big bluffs I don't like, works but get called a lot too. Top pair top kicker is worth betting though, ie go for thin value, reraise means nuts.
If you get reraised, need to beat random 2 pairs or good outs to it (huge implied odds)
You can go all-in at turn or river for value with strong hands, top pair + will call you
Pay a lot of attention to small stacks and their potential tendency to go all-in, they're all different but top pair good kicker is all-in on flop for sure. They can go all in preflop from BB with KJ and worse if bored or whatever.

Google up upswing multiway and blackrain79 multiway, you'll find loads of valuable info.