If you're getting juggled with, the best to do imo is to use bombs. If timed correctly, you can cancel their attack and sometimes you can even retaliate with a back kick(sorry I don't really know the correct terms). This is one of her biggest flaw ever since past games, and usually you should just try and get out of it, rather then hit the opponent. I personnaly think that bombs are vital when it comes down to surviving. Use them to your advantage to break your opponent's combo.
Samus is a very technical character(actually even more then she was in Melee), so you'll have a hard time against any fast characters. Samus has a wide arsenal of long-ranged attacks, and you should definatly abuse of it. Whenever you face opponents that you know are just gonna play with you(Sheik, to name one), try and play more defensivly, using missles, bombs and charged shots. Altho this isn't Melee, I used to play with two friends a lot(a Sheik and a Falco user), so I was constantly getting owned if I even dared close-fighting with them. I do not mean to avoid it, but try and spot the opportunities before you run in action. Like I said she's very technical,. and if you just run into the action, chances are you'll get wiped out.
Her charged shot is another one of her advantages(altho it was nerfed). Try and charge it whenever you get the opportunity to(whenever your opponent is too far to attack). But you also gotta learn when's the good time to release it. Try to use it when they least expect it, not when you're facing each others doing nothing. Being unpredictable with it can make you deadly. I personnaly prefer to use it on people in air, like when they're trying to recover. It's also a good thing to use it in relativly close range, so that the opponent doesn't have time to react. As for missiles, I'm still not very used to them(super missiles were WAY better back in Melee), but try and shoot a few of them whenever the opponent is far/recovering. You can easily prevent someone from coming back to the stage with them.
Against multiple opponents, Samus tends to be hard to play with. Her down smash is a very good move to use in such situations(and even in 1v1s).
I could have added a lot more, but yeah I hope this helps. I'm still fairly new to Brawl myself, but Samus has been my main since the first Smash game, and these tacticts seem to be good ones even in Brawl.
Edit: I forgot a few details I consider useful.
Recovery: Samus has one of the best recovery in the game. If you don't already know, learn how to use your grapple beam to get back to the stage. You should also use bombs constantly when trying to come back from the sides. As for her Screw Attack(Up+B), don't overdo it. It's a good move against edge-guarders, but it should rarely be used, as it leaves you vulnerable for a "long" period of time, and it's not a good knockback skill.
ZSS: I know a lot of people consider this to be another character, but if you can play her good, you can have a big advantage. Using two characters in one can make you very unpredictable.
Combos: In general, Samus is a much better aerial comboer, just to remind you.
Wait: Against opponents that are bad at long range, a good trick is to wait for them to attack first. If they're stupid enough to stay far, spam missiles or charge. If they are running at you, wait for the good moment, and strike them with your down+A attack. This move has a fairly good range and power, so you should be able to burn them before they even think about doing an attack.
Meteor: Probably her deadliest move, learn to use her aerial down smash. It requires a lot of training to get to know the timing, but when you master it, you can finish off just about any opponent that's recovering from the sides.
Again I'm sorry I do not know all the correct terms, so I just said how to perform them. If you need clarifications or anything, just PM me, I'll be happy to help you with the best of my knowledge.
