The only thing I can think of is lifting weights. Not real heavy, but maybe like 10 pounds or so -working the biceps, triceps... you really don't need weights for the triceps, push ups will do as would some other exercises. I know EXACTLY what you mean! I am 5'6" and 120 pounds and my arms sag, and the stomach and butt are all "flappy"! For that you can do crunches or sit ups. And the butt you can just practice "tightening" exercises. I do this when I feel like it which is only about once a week

, but it really should be done at least every other day! There's a pretty good tricep exercise (that's the muscle under the upper arm that flaps back and forth when you wave - well, mine does anyway!

)if you are interested I will TRY to explain it. I can't say if it works good personally, because like I said, I don't have the discipline to keep at anything for long! But it's what I was told WOULD work.
I did belong to a gym a few years back, didn't lose any weight (didn't WANT to!), but I ended up having to buy smaller clothes because things tightened up. I stopped going and flabbed all out again, even worse!