"Knock, knock."
" Who's there? "
" Substitution. "
" Substitution who? " Did you really have to even ask?

I've heard good things about ureasample.com and Quick Fix 4.0, but a clean donor would be your best bet if you have access to one. Do yourself another favor, DO NOT USE URINE ADDITIVES. Changing the pH or integrity of the sample may not only cause your test to fuck up but there's a possibility that if the test has been " tampered " with, they won't even give you the courtesy of a re-test. Suspicion means problems -- don't risk it. Or even worse, they may ask you to retest RIGHT then and it may be supervised since you decided to go dropping adulterants into the dirty sample. Then you're screwed.
Do some research. Use substitution. Sleep at night -- and smoke AAAAALLLLLLLL day! Peace.