Upvote:3
As a preventative measure, candlewax is good (not beeswax but the common paraffรฎn wax). You can simply rub a cheap candle all over the zip teeth. Also check for loose threads getting in there - they're not your main problem by the sound of things but anything that can increase friction is to be avoided.
It's not so good as a cure, because it doesn't penetrate. You can still put it on the accessible parts, and it will help if with gentle jiggling you can get some movement.
If you suspect it's stuck, try avoiding the tab - where it attaches is a weak point. Instead try to push the whole zip pull (the part that slides along the teeth). Starting at the end or with two pulls together, you can often use leverage (like a screwdriver). This might be a good time to try a drop of oil. Personally I'd use bike chain lube. The sort with PTFE should have a preventative effect too.
If dust is likely, try to remove it, with a brush, vacuum cleaner, or (especially inside the pull) compressed air. This can be done before or after storage. You can get cans of compressed air for cleaning cameras etc. If salt is likely, whether from seawater or sweat, rinse and dry before putting away)