I've been known to pump and found some really good info on it. From what I've seen, a lot of the big girth gainers did have pumping in their routine.
Firstly, staying hard in the pump is a necessity. A lot of guys milk the tube, watch �naked people movies�, etc to stay hard. Often you can appear and feel hard while you are in the tube, but you are building more fluid than if you truely are.
Second, keeping the tube warm while you are in it is a must. This helps reduce fluid buildup and open up the blood vessels to actually expand those.
Third, pumping is not a task for the short on time. Using a pump with a pressure gauge is important, as with using low pressure with several short sets. The pressure should hang out between 5hg and 8hg - though it's really tempting to go high.
Don't get discouraged if you don't get huge the first few times you pump. It takes several weeks to actually start getting big from pumping. Guys that have been at it for years can increase their length by 50% and their girth by 100%, using different tubes, etc.
The key is several short sets of ~10 minutes, and some jelqing in between sets. Some have had great results with a pump/clamp routine.
Hope this helps, if I can answer more questions I'd be happy to.