I've used zip ties before but most of the pouches I use utilize MALICE clips, and the ones that are 2-3 inches long are tight enough around your average web belt that they don't really slide anyway. Could always apply a dab of clear silicon caulk to the insides of the clip also to give it extra traction.
If you're using pouches with actual webbing straps on the back, poking a hole through the edge of the pouch and through the belt (with an awl or a sharpened piece of coathanger) and then threading a small zip tie through and looping it works nicely. Just make sure you keep the lock facing out and cut the excess nice and close so you're not poking/scratching yourself with it. I've modded LOTS of web gear this way, it's easy to do, holds well, and is easily undo-able if you change your mind.