I have had mixed results w/ Viagra- best luck w/ 1/2 tab 1 hr prev and another 1/2 tab 5 minutes prev, but sometimes this doesn't work so well.
Haven't tried Cialis or Levitra. I know this is
highly variable, but what else have others tried and with what kind of results.

The best person to answer your question is your doctor.  I am afraid you will find most natural forms of ED medication are crap and many are dangerous especially if used in conjunction with the real thing.  Viagra does have a generally higher percentage of side effects then Cialis or Levitra this may be something to discuss with your doctor as well. I hope that is how your viagra was prescribed and there is a reason your taking it that way( like your doctor told you to). I am assuming 100mg pills, if so your dosing yourself.
Doc

p.s.  Anything I said about meds can be found in the  2009 PDR

I'd take 25 mg. with morning coffee on the day of the appt. and 75 mg an hr. before the appt., usually on the road to the appt., where I get a call canceling the appt., or there's no response to my call. Then I sigh and count the cash I still have and go have tempura at my favorite Japanese restaurant--sometimes I'd do Korean over in Downers Grove. Go home and have to explain to my wife why my face is so flushed.

greeted by your wife with your briefcase hanging on your erection?