How can I prepare left over Stir Fry for the fridge?
I make stir fry all the time and do the same thing as you. I would make extra, enough for 2 - 3 meals. 

The thing I do to prevent the vegetables from getting too mushy in the refrigerator is by cooking the stirfry about 75%(I make sure if I am doing this that the meat is fully cooked first) and then take out the portion that I intend to refrigerate. This allows me to either finish cooking it in the microwave if I am taking it to work or reheat it up in the stove if I am staying home.

Also since I am taking the portions from the upper part of the stirfry, there is less sauce that the vegetables will be steeping in so that should solve your problem of too much sauce.

Another thing you should remember is that the types of vegetables you use makes a big difference. If you choose all vegetables that turns really soft when cooked(onion, squash, peppers) then reheated stirfry using those vegetables will be not as unpalatable. The trick is to use a variety of textured vegetables. 

Also I dont want to state the obvious but if your problem is your sauce, have you ever tried using a different(lighter) sauce recipe?