I can sympathize. We do several things;
1st I always bring my wife to 1-3 orgasms using toys before any serious penile stimulation
2nd I use Cialis which in addition to it's labeled use of enhancing erections - it helps me prolong sex by delaying orgasm
3rd I've started using a simple silicone cock-ring which helps me stay hard for a while after orgasm. My wife says that the ring also makes my erection noticeably larger - nothing drastic, nicely noticeable for her.
Hope this helps