Assuming San Diego will be like the All Star Game in Anaheim a few years back, we waited until a few days before and actually got tickets to Home Run Derby for under face value, and game tickets for face value. I think folks panic right before and the prices dip a bit - San Diego doesn't have near the local population pool to draw off of that Anaheim did, and while it is a beautiful city it is an expensive place to fly into (not a ton of flights). I'd imagine there will be tickets available up to the last minute for not much over face value.
Feel free to PM me for any notes on hotels, touristy stuff etc. I am down there 15-20 days a year minimum, and go to 8-10 games a year at Petco. PG Staff is right - Phil's BBQ is darn good, and we have 5-6 other favorites we visit down there throughout the year. Beautiful stadium - highly recommend visiting the city and the ballpark, you won't regret it.