Other Art Supplies
You should be spending most of your money on the paint, brushes and the painting surface, in that order. However, these are a few extras I've discovered that will save you a lot of time. If you've used your beginner supplies and decided you like painting, it's time to start investing in some extras. This is a list of supplies which are not necessary for beginners, but are things I use every day as a professional artist and couldn't live without. I recommend getting these things once you start buying professional quality paint, since each will save you money in paints and brushes.
Palettes
1) Disposable Palette Paper ($5)
You can buy a pad of disposable palette paper, some with a hole in the pad so you can hold it with your thumb like a traditional palette. The paper is waterproof, and when you're finished you can peel off the used layer and throw it away. This is the quickest, easiest and cheapest solution if you only paint occasionally. Although you're throwing the sheets away after one use, it may still be better for the environment than a traditional palette you have to clean with a lot of water. Depending on the humidity of the climate you live in, I recommend spraying the palette once in a while with a spray bottle just to keep your paints wet.
2) The Sta-Wet Palette ($15-20)
The Sta-Wet palette is a resealable plastic tray that has a sponge inside and a special magic water-permeable paper liner. When open, it will keep your paints wet all day, so you can paint for several hours without your paints drying up. When finished for the day, you can put the lid on, and it will keep your color mixes wet for weeks. I find this cuts down enormously on my paint consumption and saves time.
The Sta-Wet palette is great for acrylics. I used to use disposable palette paper, but I switched to a Sta-Wet palette when I went pro, and I wish I had done it years ago. While I still recommend disposable for beginners, travelers and occasional painters, the disposable palette results in a lot of wasted paint since the paint often dries up before you can use it all. Every day, you have to squeeze out new paint, which takes a while if you have a lot of colors. The professional quality paint I use is expensive, and I'm sure my Sta-Wet palette has paid for itself.
A Tube Wringer ($10-20)
I use expensive professional quality paints and tend to go through them quickly, so I'm always looking for ways to avoid wasting paint. A tube wringer really gets all the paint out of the bottom and helps the paint tubes fold up smaller as they get used up. It's also great around the house for toothpaste, etc. There are small painters' keys that do the same job, but they don't work nearly as well.
A Brush Washer Tub ($6-12)
I held off on getting a brush washer tub for years, because I wasn't convinced that it would be any better than using an old jar. However, it's worth the money once you get serious about painting. The bottom has a ribbed area where you can scrub your brush bristles, and it gets paint out of the center of the brush much better than anything else. The holes on the sides are handy for sticking your rinsed brushes in, so you avoid the temptation to leave your brushes in the water. I'm generally pretty rough on my brushes, but the brush tub has helped extend their life expectancy a lot.