A couple of things that have worked for me.
Tribulus- I grab a bottle for $20 from gnc that is 500 mg standardized at 45%. I take 4-6 pill for a few days then 10 a day for maybe 5 more days, then I stop taking it. 3-4 days from the start of taking it, I notice a fuller harder erection that doesn't fade easily. This effect last for about 10-14 days. Usually I lay off for a few weeks then start over.
Also, L- arginine- Its funny why I bought this in the first place. I saw a $25 bottle of Some kind of sexual stimulant at the pharmacy. I looked at the ingrediants and they were L-arginine- 2500mg and calcium??. So, right next to it is a bottle of L-arginine for like $6. Anyway, I've tried 2500mg about 2-3 hours prior and it adds a nice fullness and it seems to add a little different feeling to sex, a good feeling. It hard to explain.
Either way, here are two relatively cheap and tested ways to help you out.