My suggestion is you wash your gi in cold water as soon as you get home and then air-dry it. Every 10 or so washes I put mine through a HOT wash cycle - it works for me!

buy a black one

Make sure you use a biodegradable eco-friendly wash powder. Better for you and the environment.

Ha ha ha ha, good one Shane. A couple of flippant suggestions first: 1) Don't take it out of its packaging and 2) don't ever use it; and I guess for some guys the answer would be, "Give it to a) mum, b) the girlfriend or c) the wife." Clearly, these guys would be in definite need of a self-defence system.

Here's my practical advice, other than invest in more than one gi and rotate regularly so that you can be cleaning and drying one without disrupting your training schedule. The part that get's the grubbiest the quickest seems to be the inside of the collar. These days it does anyhow, as when I first started my training it was the back of the jacket that got the dirtiest as 100 or more sit-ups per session was not unheard of, whereas sōji was unheard of.

It may be impractical for some folk, but just grabbing a shower before going to the training hall can help to keep the inside of the gi clean. I once trained with an instructor back in the UK who would get to the YMCA early and shower before each class. And that was on a Saturday afternoon, not straight from weekday work. Similar ritual to aikido-ka who are obliged to wash their feet before stepping onto the mat. Choosing the right under-arm deodorant that won't leave unsightly yellow stains can also help. Conducting sōji is also helpful but not always possible, especially in a school hall with heavy foot traffic. A cursory dry-broom sweep up and down the hall does little to remove the stuff that's going to stick to your gi.

So, here's my list of basic essentials. Sunlight Soap (bar), Napisan, soap powder, hot water wash. The Sunlight Soap is an old standard and very effective too (my wife Kim showed me how to use it properly so credit where credit is due).

Method: Place garment in the sink or wash-tub (leave the plug out) and run some warm water over the effected area to dampen it. Keeping the gi on a firm surface take the bar of soap and rub it vigorously on the dirty area until you get up a good lather. You can actually see the dirt disappearing as you do this. Let it sit for about 15 minutes then into the washing machine, soap powder of your choice (you can also grate some Sunlight soap into the mix as well, use a DRY bar of soap and NOT your kitchen cheese grater), a dose (i.e. one cap-full) of Napisan, hot water wash cycle (not every time) and, Eh! Voilà, one clean gi.