I just had a set of rims made up in 19/17 sizes and I decided to make them tubeless. I've put up a page showing how it's done on our site so feel free to check it out:
For those who feel it's too much work to click that link I'll give you a brief summary. Take your rims and scuff them with a scotchbrite pad to put a tooth on them - this is essential. Then clean them with alcohol to make sure they have no grease or oils on them - this too is essential as you want to make sure there's perfect adhesion.
Go to your office supply store and get a package of 3/4" dots - orange ones work best for obvious reasons but I had to use purple which is unfortunate. On the back I went with larger 1" ones but I think in retrospect I'd stay with the smaller ones. They don't have to be perfect you just want to keep the nipple free from the silicon you're going to slather on there so if you ever want to adjust the wheel you can. I've never adjusted a spoke in my life but I have it on good authority that there are people that do that.
This time I tried a very fancy and expensive aircraft epoxy from Aircraft Spruce
that is used to seal gas tanks. I thought that this would have better adhesion and be more resilient and I think it would be but I went too thin on the rear wheel (2-3mm) and it pushed the sealent through and leaked. I didn't want to mix the stuff again so I just covered it with regular household silicon caulk. It's holding fine now. Since that sealant is so expensive I wanted to spread it over two sets of wheels and in retrospect I should have used it all. Now you know right? Mix the whole contents of both together and you'll have more than enough for one set of wheels. Now spread the stuff (wearing gloves since it's nasty stuff) into the center of the wheel keeping it off the bead area. Use a plastic scraper that you cut to fit that gap to evenly spread the sealant of choice over the spokes and go for about 5-7mm depth. If you use silicon you have only 10-15 minutes before the stuff starts to skin and smoothing it after will make it worse. The aircraft epoxy has a pretty long working time (an hour?) and is about the same consistency. Both can be cleaned with alcohol while wet. I've always done this on a balance stand because I have one but you could mount the wheels back in the bike and block it up on the centerstand.
Before you do this order some metal tubeless valve stems that bolt in - they're more durable and easier. If you want to be trick get the 90° ones for the front and point it to the side without the disk if you've removed one.
That's it. It works. It's not rocket science. I learned this from Kenny Buchanan so the source is solid. I have a few friends that race supermoto and they're all using the aircraft epoxy.
I first did this 6 years ago with silicon on my boxer cafe bike and it's still holding air - better than any other bike I have I might add. If you use silicon you'll need two tubes for two wheels.
I've mentioned this in other threads and I believe Woody has as well but I don't recommend this for off road or for the 21" Conti TKC80 since it's a flimsy tire and I've had it lose air and roll right off the rim. I'm sure if you're using the bike with stock rims and tires on pavement you'd be fine.
Now, I'm going to ride my bike...