Is it possible/sensible to add a suspension fork to an old mountain bike?
I would consider looking at a used later model bike.The improvements made in the last 18 years are worth the money.A decent fork can run hundreds of dollars not including installation. Check with your local bike shop for used bikes or craigs list if you keep it local so you can see before you buy.Bikepedia is a good reference to make sure you have an idea what the bike cost new. Also check the minimum inflation recommended for your tires and run them softer than you have been,this will cushion some of the bumps.An alternative might be to install the biggest 26 inch tires you can fit in your frame and lower the tire pressure to get more shock absorbtion.Be careful of really lowend bikes from the big box stores Walmart etc. a really cheap shock with no dampening or adjustment is worse than no shock.