Some of the decals on my tired, old sails are starting to come off and my boat's looking a little ratty due to this.
What's the best way to re-adhere the 'cals so we can be sharp-lookin' for the season?

Thanks in advance,
Nick

Are you talking about things like the manufacturer logo and the sail numbers? If they're that old, chances are you could stand to buy a set of replacement sails anyway!

If you still just want to restick them, you can call just about any sailmaker to have a set remade from sticky back dacron, go to a vinyl shop (any one will do - although vinyl will probably not adhere well to an old dacron sail) and have some remade, or purchase a can of spray mount adhesive from a craft store. The spray adhesive is probably your most economical alternative and should last a while. Remove the entire letter from the sail, spray the adhesive on the back (using a piece of scrap wood or plastic underneath as there will be a lot of overspray), let it dry for a few seconds, and place back on the sail while it's still a little gooey.

Most spray-mount adhesives are intended to be applied to both surfaces, allowed to dry, and then applied much like contact cement. You can't spray the sail because you'll end up with a big sticky, and eventually black/dirty, spot on the sail. To compensate, apply the number while it is a bit gooey shortly after spraying rub it in place gently and let it sit for a while before moving.

aaahhhmmm>??? new decals? I can help you in that department.

What kind of material are your sails? You can probably get away with using some type of RTV silicon and paste them back on. Make sure that if you choose this route, you apply a very small ammount around the edges of the decals and smooth them out with your fingers. Should work good, if let dry for 24~48 hours.

Good luck.

edit: and what Jake said as well! Good recomendation in his post.