Okay frist, part of it has to do with age
I maybe wrong but I reacl seeing studies where being older ment more difficulty getting and keeping errections
Like over 50+ to 70's+
Anyway a good method is a tight cock ring
and tieing off your balls to hold the blood in the cock
however it is also dangersous becasue restricting blood flow to any part of your body is dangerous.
A strong urge to pee can help you keep an errection longer too, but will have the opposite effect on getting one in the first place
A large dildo may help but I am not sure if you consider that physical stimulation
I have used vibes on just my ball but again you may consider that stim.
The best way is actual MINIMAL stimulation
http://www.milovana.com/webteases/showf ... p?id=10020
may help you with the idea of how to do this.
one stroke on your dick every five seconds or so
can help you maintain an errection
or you could also slap or tap it every so often just enough to keep it hard.
There are warnings about certain drugs that can cause you to keep an errection to long
but I have no idea exactly how they work and do not use them.