Assuming cash games and that you're new to poker, and are playing NL holdem:
Stop limping.
Never play with a stack less than 100bb.
Stop calling so much pre-flop.
Start stacking more lightly when the pot is big relative to the stacks.
Improve your table selection.
Download Holdem Manager or Poker Tracker 3, download pokerstove (the latter is free).
Stop set mining out of position and in 3bet pots.
Pay way more attention to position that you already do.
Play more
hands against the fish, and fewer against regs.
Use more conservative
bankroll management.
Just to rattle a few points off of the top of my head.