The draconic breath feat says "If the breath weapon of your draconic forebears does not deal acid, cold, electricity, or fire damage, you choose from among the four energy types mentioned above." Does that mean I pick one when I get the feat or each time I use it?
I'd suggest that you'd pick the type upon choosing the feat, otherwise it'd be mechanically better to choose a draconic forebear which doesn't deal one of the energy types so you could get the extra utility.
This DM would allow any creature that possesses the feat Draconic Breath to pick anew the breath weapon each time the feat's benefit is used
The benefit of the feat Draconic Breath (Races of the Dragon 102), in part, says
As a standard action, you can convert an arcane spell slot into a breath weapon. The breath weapon is a 30-foot cone (cold or fire) or a 60-foot line (acid or electricity) that deals 2d6 points of damage per level of the spell slot you expended to create the effect.
To this reader it seems that, prior to the arcane spell slot's conversion, no decisions about the feat need be made. That is, before the creature takes the standard action and expends the spell slot, the creature has no breath weapon (at least via this feat, anyway). Nothing about the feat's benefit seems to this reader to limit the creature to only ever one of the listed area types then one of the corresponding energy types. In fact, this reader has always imagined sorcerers were just versatile that way, tapping into the power of dragonkind broadly to produce whatever effect they need at the time.
Likewise, to this reader the feat's Special entry seems to encourage this reading, saying, "If the breath weapon of your draconic forebears does not deal acid, cold, electricity, or fire damage, you choose from among the four energy types mentioned above," and again failing to mention when or even if such a choice need be made. The Special entry to this reader seems to be just a reminder that, for example, having a pyroclastic dragon in the family tree doesn't entitle the creature to an off-menu sonic breath weapon via this feat.
In short, this DM would be okay with ruling that the creature can pick a different (or the same) breath weapon each time a standard action's taken to convert a spell slot to activate the breath weapon. This DM believes that feats are allowed to do things, and to this DM that seems a reasonable thing to allow for a two-feat investment that also costs precious—and, to stay relevant, increasingly higher—spell slots.
However, this player can imagine another DM seeing the feat's benefit as potentially unbalanced if read this way. Being able to pick from among even four energy types does give a sorcerer—and, let's face it, the feat will likely be in the hands of a sorcerer who has plenty of spell slots to expend—a degree of versatility that a DM may not want or expect. This player would try to convince such a DM that the two-feat investment is, therefore, excessive for the result—saying, for example, that a sorcerer 6 can pick the spell lightning bolt—that has double the breath weapon's area—and save two feats, and that spell remains relevant until level 10! But, were that DM to remain unconvinced, this player wouldn't leave the campaign over it—there are plenty of other good or better feats; nonetheless, were such a ruling handed down early in the context of a potentially long-term campaign, I might either pick a different feat or retrain away the feat after level 6 or so (when the feat's largely no longer relevant) were the Player's Handbook II feat retraining rules in use.