What the others stated above should definitely be checked. This may be obvious, but just in case, and nobody mentioned it, always tune up to pitch, not down. If you're slightly sharp when tuning, bring it down (flat) and then back up. If you tune down to pitch, then bend or use the trem, this would cause the guitar to go out of tune as well. Also, may sure you stretch your strings. Sometimes, when I tune my PRS DGT Std, it is slightly sharp. Before touching the tuning peg, I pull up on the string. Oftentimes, this is enough to get it to pitch. If this doesn't work, then I loosen the string and bring it back up to pitch. When I'm just about done tuning, I always pull on the strings, stretching them a bit. Most of the time, I'm still in tune, or maybe one string is now flat. Seems like a lot of trouble, but it's worth it. After an hour of playing, I'll recheck after a break and usually the guitar is still in tune. I use a strobe tuner too. Sometimes, a week later the majority of the strings are still in tune. That's why I love PRS guitars!