Best Procedure to Tighten Loose Skin on Upper Thighs?

The problem you describe is tough because you are too young and in too good shape to tolerate the scars from a thigh lift.  These are generally reserved for massive weight loss patients for whom the scars are a reasonable trade-off for massive excess skin.  Also, the non-surgical treatments like Thermage are quite disappointing and not worth the money and pain.  All most people can do realistically is get a personal trainer and add strength to the muscles underlying the loose skin and drape the skin over more muscle volume thus thightening it.  Also, avoid serious sun exposure as this will only worsen your elasticity over the next 15 years.

You have alluded to the answer in your question i.e. if the thigh and buttocks sag the correct treatment is to lift them. Without photos or a face to face examination it would be impossible to say what treatment approach would be best or even if surgery is a good option. I would be reticent to perform a thigh buttock lift in a 30 year old who is not a massive weight loss patient because patients who do not have a lot of excess skin like after massive weight loss are never happy with the scars after a thigh buttock lift. The scar trade off is better tolerated if a lot of hanging skin is removed.

The thighs are a difficult area to treat.  If you just have fullness, then liposuction may be the best option.  However if you have loose skin, then you likely need a thigh lift.  The incisions can vary widely and clan only be determined by examining you directly.  If you post pictures, we may be able to give you more specific advice.  Good luck.