I just saw this thread and wanted to give you my experience with these creams. I was put on clindamycin phosphate and tretinoin .05% about 7 months ago by my dermatologist as well. I was having a pretty bad breakout that literally popped up out of nowhere. He told me to use a gentle cleanser (I use cetaphil) in the morning and then put on the clindamycin and then use the tretinoin every night. As mentioned above, tretinoin works by pushing all of the acne out to the surface of the skin. My skin "purged" pretty badly from week 3 to about week 6..but after week 6 my face gradually stopped giving me those huge, disgusting pustules.
It's good to ease into the cream, but I urge you to eventually use it every night. Don't stop and don't skip days. I also use a moisturizing sunscreen because tretinoin makes you REALLY susceptible to sunburn. It's also normal to get really flaky skin/ itching skin in the first few weeks. The best advice I can give you is just try not to give up on it...because there may be times that you really want to. Stick with it and give it a few months...it will not work overnight. Good luck!