1.) Use two water cups for painting. One for rinsing off your brush, and one for putting down paint on the paper and prepping the surface for washes.
2.) Mason jars work well. High top jars/cups tend not to work as well because you have to make more of an effort to dip the brush in. You could even use sauce dishes that are shallow and small.
3.) To prevent unwanted bleeding, let the layer dry first.
4.) The key to concentrated color is layering your washes/paints.
5.) Use good brushes. Bad brushes tend to have tips that are not very precise and when doing delicate details it will be very annoying to deal with the stray hairs.
6.) There's no need to spend too much money on paints and paper. There are good quality materials for you to buy that are decently priced and won't break the bank. For paper I recommend Rives BFK and for good watercolors I recommend Grumbacher or Van Goh watercolors. If you're an experienced watercolor painter I recommend brands like Holbein and Winsor and Newton.
7.) Combine mediums. Good examples would be colored pencil and ink/ink pen. The colored pencil gives you control that you wouldn't normally have with capricious water medium and the ink pen can give your focal figure a bold outline if that's the style you're going for.
8.) It's ok to work small with watercolor, especially with starting out.
9.) Don't drink while painting. Nuff said.
10.) Be careful of dark color mistakes because they can be hard to cover up or get rid of later on. Watercolor is like working with multiple layers of colored plastic sheets in that sense.
11.) Try unusual color blends. You can make very beautiful and unique looking colors by mixing combinations you are not familiar with.
12.) Keep in mind watercolor acts differently from acrylic and oil. You don't "scrub" colors together with watercolor. You let the water "flow" and blend to create the painting.
13.) Have your paint pallet and water jars close together so your hand doesn't have to move so much (even I have this problem sometimes).
14.) Try to work quickly while your paint is still wet to get in those little color "spreads" and "diffusion". They can make a scene look gestural yet intricate.
15.) When you make a mistake blot it with a paper towel or a tissue for a quick fix or if that doesn't work blot it with some clean water (a small amount) while it's still wet to get rid of it. If it's already fully dry it's going to be a lot harder.