Use Sock garters, that is what a lot of men do.
There is not a very straight solution for making your old stockings tight again, however, next time when you will buy stockings keep in mind that you buy the larger size of stockings. Basically currently you are wearing stocking of short length or you are not buying a good quality stocking. It is better to buy long one next time. And if you wear pants then you can use a thread to tight old stockings.