I played a frost mage in Battlegrounds back on Deathwing when I was hardcore catassing WoW. I also played escort heal bitch for a good friend of mine who was also a frost mage who liked to run the flag. Here's some pointers:
1) At level 29, you're not going to really have the luxury of mixing talents across trees if you want to do some damage as a frost mage in PvP. Frost PvP damage is completely crit dependent and largely based around the ice shards/frostbite/shatter mechanism. Players get rooted by either your frost nova or off a melee hit (frostbite procs on melee hits) , you can turn and then hit them with a frostbolt and a cone of cold before the root breaks, and both spells will have a very large crit rate and very high crit damage. From range, it's just about spamming frostbolt mostly-it's a better PvP spell than fireball with talents since it both snares and has a shorter casting time than fireball does.
2) Frost mages benefit greatly from +damage gear in terms of being able to go from simply harrassing players to mowing them down. Don't worry too much about having super killing power without it.
3) Cold Snap and improved frost nova are two of the better flag-running talents. Cold snap will allow you to get an extra frost nova off, and if a one-on-one fight is close it can also provide you with an extra cone of cold for another instant to finish a player off with.
4) Forget arcane explosion. It eats up mana like crazy and does very little damage. Only useful for finishing someone off when you are near a rejuv hut.
5) When flag running, watch for dispells against your ice armor. It's a pain to keep it up since it costs so much mana, but without it no healer is going to keep your butt alive if you get stunned by a rogue or what not.
6) When running the flag, always keep moving forward. Use blink to get separation in that battleground liberally, preferably right after rooting a few would-be chasers with a frost nova. Mana shield isn't a bad thing either, but note that the ice barrier you'll get later on in levels as a frost mage is 100x better.
That's my suggestions. Oh, and find yourself a healer type if you can, preferably a paladin for blessing of freedom to keep you snare free while running the flag.