All of my gig money goes in a separate account. That's my gear fund, plus it's easier for taxes at the end of the year because I can log the activity in that account and *boom* I have all of my expenses that I can write off.
Now, I didn't buy my gear all at once. My custom ESP was a purchase made in 2001, when I was young and had more dollars than sense (not that it isn't an amazing guitar, but I should've went boutique). Sold off all my old gear to afford the Axe FX and gig money covers the rest. Honestly, most of my guitars were acquired when I was young because in my area I had a high-paying job at 18yrs old ($9.50/hr, but rent was only $300). Oh for the days when housing costs were a small fraction of income!