You should get the best letters from those who meet most (or all) of the above criteria.
Also, be sure that you give your letter-writers plenty of time before your deadline (e.g., three to four weeks).
And, give your letter-writers an organized packet of materials (e.g., pre-addressed and stamped envelopes, all necessary forms to be completed, any specific instructions that need to be made clear, obvious deadlines for each letter to be written, etc.).
The more you do (in school and out), the more your letter-writers can write about, AND the more impressively you will come across.
You WILL NOT get strong letters if you do any/all of the following:
Should I waive or not waive my rights to view letters of support?
In my opinion (which, I promise, is shared by many), when you do not waive that right it means that
the letter-writer knows you might see what was written which would mean that they might not write as
honestly as would be the case if they knew the letter could not be viewed. Probably you would be
asking people who would not mind you seeing the letter, but that isn't the issue. The issue is that
the folks reviewing your application cannot tell which type of letter-writer they are reading from.
A letter that is in the "waived" category could be weighted more heavily than a letter in the "not
waived" category.
FYI - I will not write a letter for you if you do not waive that right.
WAIVE!