since I haven't seen you play, I can't give many tips unless as a general statement; try to not be stuck in shield. while shield is one of the most powerful tools in s4, grabs -- or even the shield stun on sufficiently positive moves-- can completely neutralize them. are there any particular characters that you struggle against as a whole?
try saving replays of the matches you do. go back and watch them and see why you get caught by their moves. of course, it also depends on character attributes.
i feel that heavies are often easier to combo than floaty characters, for instance, samus, Luigi, or peach. who do you play?