For some of the really, I mean REALLY light guns, like the Crosman 2100/Remington Airmaster 77, practically all of what little weight there IS is forward of the trigger. Okay for some styles or tastes, but kind of icky for me .
I used a hole saw to "drill" a 3/4 inch hole in the "buttplate" of my 77. Then, using good quality modelling clay (my wife's an artist) I rolled a bunch of grape-sized balls of clay and dropped them into the hollow stock. About every 10-12 "grapes", I smashed them down with a 3/8 inch dowel rod, just to press the air out. The clay is a little heavier than a similar mass of walnut, but with the inevitable voids, it comes out pretty well. I figure for a $50 gun, it's a pretty good improvement. The gun is still remarkably light, and because the balance (to me) seems more natural, I don't actually notice it feeling "heavier".
I did this to a 1077, also, but I think I was able to unscrew the buttplate from that one first.
So far (18+ months), the clay hasn't dried out or broken up - but again, its good quality, "non-hardening" clay. I wouldn't use Play-Doh or similar, unless you have a trasher-gun that you just want to practice the technique on. ...of course, even if you did "ruin" one of these stocks, you can get the replacement plastic from Crosman for next to nothing...
I also have a Walther 1250, and it's hollow, but I don't think I'll mess with that just yet
*edit* - Forgot to mention, I got a hard-rubber "furniture plug" from the hardware store that fits the hole I drilled. It plugs the hole perfectly, and sits pretty flush - still really obviously there, though, but this guns a plinker, not a wall-hanger!