I used to have that problem and I have 20+ projects that were started because I saw a TV program or an article in a magazine that I just had to start right then. Before I could finish someone would ask me to make them a quilt and I would put my project in a bag or box and never get back to it.
My solution to the problem was I started a project book. When I come across some project I really like instead of grabing fabric and start cutting, I grab my pens and my book and I draw the block or quilt idea, I include any fabric ideas and my thoughts about how I would like to see this quilt done. If I have pics or instuctions or fabric swatches I include those too. This satisfies my creative excitement about the project for the short term and it gives me a great resource to turn to when I someone wants me to make them a quilt.
I also took all of my unfinished projects out of their bags and boxes, made sure all the fabic and instructions were with each project and placed them in individual clear plactic sweater boxes and placed them on a shelving unit that I can see when I walk into my sewing room. So now when someone asks me for a quilt I go to my unfinished projects first to see if I have a project that would fit what they want.