I have the same problem. What I did to handle it was to start writing down an outline or synopsis of the new story ideas as they came to me. This allowed me to get them out of my head and store them off somewhere for a while.
I usually have at least two, and usually three, projects going at any given time. Once I finish one, I will go back to my story pile and choose whichever one strikes my fancy the most. I find that by doing this, I am usually concentrating on the two or three most interesting ideas at all times.
Also, I will select one of those ongoing works as the primary, and it is usually a full length book. The one or two side projects are usually novellas or short stories. If I know I'm going to have an extended amount of time, I will work on the longer work. If I am limited for time, then I will focus on one of the shorter works.