My brother runs a couple of marathons a year. He usually gets on a pretty tight routine, ramping up the miles, mixing up LSD runs with speed work, and on race day he's right where he needs to be.
But sometimes, real life gets in the way, and he doesn't feel like getting up that extra hour earlier than normal, just to run by himself in the dark. Or after a long day, kids' soccer practice, projects around the house... he just can't bring himself to hit the road after all of that.
So what he does is a time-honored tradition among weekend warriors: he drops a load of dough on the most overpriced piece of gear he can find, and then he leaves it out in the open so that he has to look at it all of the time.
You drop $150 on running shoes, and you're not going to let them gather dust in the closet.
Of course, this is only really necessary if you're at the end of your rope. Me, this year I'm going for the ascetic, monk-like approach. I started the summer with new bike tyres, and I'm not going to add another thing until the first snow, I've promised myself. There's a small rip in my ten year old seat bag — I duct-taped the seam from the inside and it's as good as new. Repaired my shoes with Shoe-Gu, so they'll last another 2-3 years. Felt like my cleat/pedal connection was a little messed up, and REI had Crank Bro pedals on sale. Lordy, they were tempting, but instead of buying new, I tore everything apart and repacked the bearings, lubed everything, and I'm pretty sure they'll last another couple of years. Best of all, I got a flat, and instead of replacing the tube, I patched it up and put it back on. Of course I did it wrong, flatted again, and had to do it all over. But even still, saving that one tube has a grace all unto itself.
So choose your poison. Even though the two methods are diametrically opposed, they both work.