First I vacuum any loose dirt. Then I wrap a wet rag around the end of a flathead screwdriver and clean the rest.

I also brush it with a good brush then vacuum. If you do this often it is not hard to clean.

I do the same with my window sills, I use a small awl or screwdriver to keep the drainage slots open so rain water can run out.

I got a detail brush set at B,B&B and like it.

Here is a link that might be useful: Detail brush set

Brush then vacuum. I really wont recommend getting any liquid down there. There might me parts of the sliding window/door that could get exposed to rust. As for how often, well that depends on where you are. If you are in a dust/dirt-free-ish then every month or two should be fine. If you're somewhere where dirt is a threat then that's a different story.

Window tracks...ugh! But an old toothbrush will be your best friend. I vacuum first, then use the toothbrush to loosen the stuck on dirt. If it's rained and the dirt is now mud, don't vacuum! It is time consuming, but it works well. Then, I follow with a lint-free cloth or paper towel and a bit of spray cleaner (like 409) to get the last little bit of residue. Once this is done, regular vacuuming will help to keep it from building up so badly.

A handheld steamer and a small shop vac worked great for me. quick and easy. Follow up with a damp microfiber cloth, and things are spotless!

I will never get white again because of the track. Mine looks dirty when it is not. They used a sealer when they put the door in and it looks dirty even when the track is clean.