try to get a LOR as early as possible especially the ones that are science related.. that way you don't have to do it in the last minute and panic. Also, see if your career services dept at your school has a place where you can store all your LORs. The great thing about storing it is that you don't have to bug your prof if you decide to apply to another school, you can just call them to send it out to the schools, and they store the LOR for 4 years. The only bad thing about that is that some schools have their own forms that they want your prof. or opt. to fill out....
and I agree.. make sure you have everything prepared for your prof or opt before you meet with them. An addressed envelope with stamp, forms, and maybe your resume or personal statement if you aren't too familiar with them. Oh and remind them that their letter has to have the proper letterhead.. whether it is the school's or his/her private practice!...