Well a pump is exactly that.. It's when you're training a muscle, that muscle becomes engorged with blood ad waste product (lactic acid) which is temporary and needs to be flushed out of the tissue. Lactic acid is what gives you that burning sensation at the end of a set - it's a way of 'shutting down' as to avoid injury (though obviously, we try to push past that).

Like I said, the pump is short term. Maybe lasting half hour to two hours, depending on variables. I don't think you should be using steroids or performance enhancers just to feel a pump in your muscles longer.. that seems more like a problem of the mind, rather than a problem with the body. What I'm saying is, you should be working to grow your muscles, not just substitute that feeling with a pump.

But, to answer your question. There's a number of compounds that will prolong your pump and/or give you that pump upped feeling. For safety sake, I won't mention certain compounds, because using them for a pump is just dangerous (i.e. insulin).

Certain orals like Dianabol that increases water retention inside the muscles will prolong this pumped feeling, and depending on dose and what other drugs you use, will make you look as if you have a constant 'half-pump' going on. Injectables like EQ that increases RBC will aid in that pump look/feel too.

But OP, you are doing nothing wrong. Pumps are temporary. If you had a muscle engorged with lactic acid and blood (which is in the true sense a 'pump'), your muscles wouldn't eventually start atrophying. Work towards gaining quality muscle, and not feigning quality muscle.