I couldn't cum when I used viagra once...But I had a fuckin lightining rod for a cock...I gave a girl a broke down pussy that night...I stick to popping about 2000mg of L-arginine now before sex and I have a rock hard pimp cane that will go all night and into the morning...Oh and I am on Test. 
If I am fucked up on various substances, I will bite a 20 mg. Cialis in half, I find that that is more than enough to make me hard and able to still come. Sober, I obviously don't take anything. I still say the most importan thing is abstaining from jerking off. Obviously not if you aren't getting regular pussy, but if you know you are going to get laid on a given night, don't jerk off and you'll see a world of difference.