Techniques for handling pumped arms?

I love rock climbing and I try to go to my local gym and boulder as often as possible (usually 2-3 times a week). But it's really hard for me to climb for any extended period of time because my arms get pumped very quickly. Sometimes my arms will start getting pumped after just 3-4 easy routes while I'm warming up.

So really I have a couple different questions:

• What should I do before climbing to increase the amount of time I can climb before my forearms start hurting?

• What should I do when my arms get pumped and I want to continue climbing? Usually I'll take a short break and bend my hands forwards/backwards like in this picture:

  and it definitely helps. I'll also shake my arms which seems to help too. Are there other stretches/workouts that will help me get over it?

• Is there a certain point where I should stop climbing because of pumped arms, instead of pushing through? (To avoid permanent damage) Where exactly is that line?