One of the most useful things I do is do test swatches and warmup pieces. Test swatches are just what they sound like. I use a single piece of fabric and try out designs so I can see how they look and how easy/hard they are to do, without the distraction of pieced blocks behind them. The test swatches usually end up a major mess, as I'll try different things as long as there's room to squeeze them in, or designs will morph from one to another. Very frequently, I'll get an idea, try it out and decide I don't like it, or make changes to the design.
After picking out free-motion quilting in two huge sections of a quilt last year, because I decided I really didn't like the design, I've decided it's well worth the time to test in
The other thing I do is warm up blocks. When I want to do mindless sewing, I make crumb blocks. I trim these up to about eight inches square, then layer them with a double layer of batting and keep them near my machine. When I sit down to begin free-motion work, I pull one or two or these blocks out and warm up by doing the same design I'm planning for my quilt. This can really help to get the rhythm going if it's been a little while since I worked on the quilt. These quilted blocks become potholders, eyeglass cases, etc.
Once in a while I feel like I need to a little extra testing. For my Triple Star quilt I've been picturing a vine with flowers and leaves, in bright variegated thread. I looked through some quilting ideas and saw a flower, then did some scribbling on paper to decide I liked the flower with a spiral in the middle. During a break from finishing the Cool Jazz quilt I tried the design and decided I liked it. With Cool Jazz done, last night I threaded up the machine with the variegated thread I bought for Triple Star and tried it. I love it, just what I've been picturing. Now I need to decide on bobbin thread and do one final test, then I'll be ready to start quilting as soon as everything is pinned.
I'm looking forward to a good weekend of quilting. More later.