You're so excited, you've just discovered the most perfect, fun-to-knit pattern in the most lovely yarn imaginable, and you cast on and go! It stays fun for about three days ... and then its slows down a little, and then it slows down a bit more, and you start looking at other patterns and yarn, or the next Knitty comes out ... and you stop. You don't know you've stopped; you just put it down for a while and you cast on for the next project.
I know. It's so damn easy to do.
How do you make yourself keep going on that project? It depends on why, exactly, you're knitting or crocheting it in the first place. If it is a present -- especially a present with a deadline -- you can motivate yourself to get it done. Bribery is the key here. Bribe yourself with images of the recipient getting the gift and being giddy with delight, agog at your thoughtfulness, brimming with smiles ... and contrast it with images of the recipient opening a wad of needles and yarn, half-knit, and saying "thank you" very quietly and putting it quickly back in the box for you to take back. Half-knit things look so bad when they are half knit.
You can also bribe yourself with promises of wonderful things that you will knit When Gift Knitting Is Finished, wiling away your time knitting on the gray, stockinette stitch boring sweater for your father, while thinking of that raspberry cowl-necked cabled Thing of Loveliness instead. Just don't drool on your father's sweater, unless you have time to wash it before giving it away.
And there's chocolate. Give yourself a piece of chocolate for every inch you knit. Simple, but it works.
But what if it's not knitted for a deadline? What if no one is waiting for it? What if it is just for you?? Scary.
We tend to put clever things made just for us on the back burner, then the back-back burner, then off to the side, and finally in the cupboard. For me, the death throes of a project came when it went upstairs. I'm still pulling things out of the stash that "went upstairs". Socks, mittens, hats, gloves, scarves, I can finish; it's the sweaters on circular needles that I can't. I suppose someday I'll just unravel them all and make something else out of the yarn, but it's still too painful. I keep thinking my hand will just wake up, and my circular needles (and my spindles and my roving and my spinning wheels) will be useful again.
If you're slowing down on your project, there are things you can do. Recapture some of the magic you had when you started the project -- write about it publically on your blog, in a Facebook post, show it off to your knitting group. When other people see it, they will love it and you will love it again too, and you'll want to finish it and show it off!
Use little pins to mark your progress. Put a pin every ten rows (followed by a piece of chocolate!) or put a pin when you started knitting each day. Those little markers tell us that hey, we are in fact doing something, making progress! Set deadlines for when you'll think you be done to the underarm, done the back, one sleeve: everything has a deadline. Before you know it, you'll be done!
Think of the fun you'll have shopping in your imagination (but be careful it stays in your imagination), getting ready for the next big project. And most importantly, think about how fun it will be to have finished your current project. Believe me, no one knows better than I how it can weigh you down having 30 unfinished projects in the background. I always envied people who just had one or two things going at a time. Now I am one of them, and trust me, it feels good!