After receiving a watch with a bracelet and needing to resize a couple more, I invested in a case holder block, a set of pins and a mallet. Overkill if you have just one bracelet to resize, but I had at least three at my house and a couple more in the family.
But some of the Seiko bracelet pins can be a real bear to remove without proper tools so I plumped down and got some tools from Horotec.
These tools really make it so easy to resize a bracelet.
This of course means I'll be on the lookout for some nice bracelet options for my collection.
Esslinger Watchmakers Tools and Watch Parts is a great place to find watch tools.
Perhaps NYCfixie has some other links; I know he's quite the modder. :-)