1. When you request a recommendation, give the teacher every possible opportunity to hint or say that he or she is not comfortable writing it. A bad recommendation will hurt more than any good one can help, so ask someone else if the teacher shows any hesitation or uncertainty about writing this letter.
Most teachers would prefer writing no letter to writing a bad letter, so offer a simple way out if the teacher is not very enthusiastic about doing this for you. For example, you might say, "I know that this is a busy semester for you, so I understand if you can't do it", or simply, "Please let me know if for any reason you don't feel comfortable writing this letter". Yet another way to assess enthusiasm is to ask whether the teacher would recommend that you apply to the schools you plan on applying to. If doubt is expressed (beyond the normal caution that the process is hard even for the best students) that is a superb signal that you do not want this recommendation.
(I got this advice, almost verbatim, to Susan Athey, an economics professor at Harvard whose
advice on applying to economics graduate schools is superb. This particular piece of advice is just as relevant to college admissions or even more so.)
2. Be very careful with gifts. Certainly, it is great to give a token of gratitude to someone who has helped you a great deal, but many teachers would feel uncomfortable with this and it can lead to some very awkward moments. While you are still a student at the school, a handwritten note on a nice card is best. For the teachers who have had a particularly profound effect on you, a gift after you graduate is much more appropriate because it will not raise any conflict of interest questions. Options include good chocolate, a nice bottle of wine (do this after you graduate so you don't get expelled under a zero-tolerance policy), or a nicely framed picture that will be meaningful to your teacher.
When you're applying to things in college (like jobs, graduate schools, and fellowships), the rules are a little different. Since many of your professors may become your colleagues or interact with you in some other way after you graduate (unlike most high school teachers), they are more touchy about gifts and it is safer not to give them to avoid the appearance of any impropriety. A nice thank you card will convey the same gratitude without any awkwardness.