The earlier answer makes the important point that it's difficult to know if you have good support for claims yet to be conceived.
You can never avoid the danger entirely, but the more preparation you put into you patent even at a provisional stage, the better. I'd say do as much searching as you can, so you know the closest prior art (though I'm not a US attorney, I've heard some US attorneys advise against this). Well, at least think about the prior art you know of. Then think clearly about the differences, and the essential integers of your invention - in effect, your main claims. Then emphasise this in the description.
The US seems more relaxed (during examination at least) on 'added matter' - but the EPO can be very strict on this. Say you disclose something that uses components A, B and C together. If you try and add a claim to elements A and C, (without B), the EPO may say this is added matter - you are 'cherry picking' a new combination that wasn't originally disclosed - and you'll have to include B as well, even though perhaps B isn't really necessary. You're allowed to spell out different permutations and variations - sometimes this is done in excruciating detail :(
Provisionals can lull one into making an unfocused, half-baked, too skimpy (or too rambling) application that isn't much use. It's more work to start with, but if you can get as close to what you ultimately want, as early as you can, you can avoid a lot of later aggravation.