What kind of paint are you using? As Webby and sunsanvil have noted, acrylics dry fast, and small amounts on a fine brush point dry very fast indeed. Depending on the application--painting eyes, for example--I'll reach for oils, when I need to apply a very small amount to a tiny area, and have the paint stay wet long enough.
Enamels will also dry, though not as fast as acrylics.
You mention wiping the brush on the jar rim. That makes me think you're dipping the brush into open jars to get the paint on them. This technique has its place--for example, when I paint my toy soldiers using gloss enamels, I'll dip in the jar. But I strongly urge you to get a palette of some kind, if you're not already using one, especially for acrylics.
I have a ceramic one, with a well in the center and six wells arranged around (Japanese, and I like to think of it as a stylized chrysanthemum). I will put a drop of the acrylic in a well, either using the dropper bottle that the paint comes in (eg, Vallejo, Andrea), or I'll use a toothpick, an old brush, or an eyedropper, for acrylics in jars (eg, Tamiya). Then I put some of my thinner (water, Tamiya's proprietary brand, for Tamiya acrylics, or isopropyl) in another well. Then I'll dip my brush in the thinner, then in the color and apply it. Sometimes I'll mix the thinner, using the eyedropper, right in the well with the color, and mix a batch for my painting session. Sometimes it's necessary to adjust the amount of fluid on the brush, after loading it with paint, by dragging it lightly over a paper towel or a cloth rag. This is something I learned to do by trial-and-error, and by watching others paint, and I tried it, till I got the results I liked.
As far as palettes go, there are all kinds of things to use. I know some guys who use old CDs. I've started using old jelly jar lids, because I can throw them out after use. I also save the little plastic containers--the size of a shot glass or so--that Chinese restaurants hereabouts use for sauces in takeout orders. I can use 'em, then chuck 'em, after a painting session. My process is still pretty much the same--I'll place the paint on the palette, then the thinning agent, drop by drop, and then mix as necessary, or dip the brush in the thinner then the paint.
If you're using Tamiya acrylics, I strongly urge you not to paint from the jar, but to use a palette and Tamiya's proprietary acrylic thinner. Tamiya's paints are forumlated for airbrushing and are intended to be thinned, and they work best this way. This, too, I learned through trial-and-error, and through discussion with other modelers. I used to paint from the jar with Tamiya, and found that some colors worked OK, others were horrible. Flat Black, for example, would clump on the brush and on the painted surface. Even after I let a coat cure, then next coat would pull it up. I decided to thin them, but I was Dutchy about it, and used isopropyl. It works to thin a lot of acrylics, but not Tamiya. They still clumped. So I bit the bullet and bought their acrylic thinner, ignoring cries from my fellow cheapskates that "it's just isopropyl, same thing". It's chief ingredient might be alcohol, but for me, it thins their paint perfectly, and straight isopropyl doesn't.
Vallejo and Andrea thin OK with water, or with alcohol, in my experience. I've been using water regularly for those. So do craft store acrylics.
I hope that helps!
