My 'simple' bathroom vanity install turned into a three day fight with plumbing

I thought swapping out the old vanity in my guest bath would take a Saturday, but the shutoff valves were so old and seized I had to replace the whole supply line setup. What should have been 6 hours of work ended up taking me 28 hours over three days because I kept having to run back to the hardware store. What's your best trick for dealing with those crusty old valves without calling a plumber?