1. Don't use retarders at all, as they change the chemical binding of the paint and sometimes never dry properly.
2. If working on location, set up in the shade or use an umbrella, and keep a very small (cosmetic size) mister bottle of water handy. You want a fine mist. Mist your palette regularly so the paints don't dry out. I have used the same palette for two consecutive paintings with out any problem of the paint drying out.
3. Put out a nickel size blob or more, and mix colors on the painting as you go. Premixing can dry out the paint. If you need to premix, add a little water to the mix, and scrape the mixed color into a pile, don't leave it spread out. I use Liquitex heavy body paints as they seem to hold their consistency longer and have true vibrant colors.
4. Rinse your brush often or add a little water to the brush, dabbing it on a rag so as not to flood the paint. Use as large of a brush as possible for the entire painting. My small brushes come out at the very end.
5. While on location, I keep a plastic jar of water half full to rest my brushes in after I have used them, so the paint won't dry on them. Water does not have to be kept clean, the paint settles to the bottom of the jar, so it doesn't hurt to keep using the same water. (I like to bring an extra jar to pour dirty water in at the end, don't pour it out at the site.) I do start a new painting with clean water, however, for pure clean skies. When in the studio, I keep a shallow water container next to a larger one. After cleaning my brushes out in the larger container, I rest my small brushes in the shallow container with a little water in it, so the bristles do not get bent standing on their heads.
6. Work quickly. Don't fuss with detail, save them for last. I lay everything in very thin and wet to get started. Then I can make adjustments easily. I like to work one section at a time. Example: get the whole sky in, then get the middle distance, etc . . . I work small when on location, so it goes very quickly.
7. Try different palettes. I like the styrofoam meat trays, they have a lower edge than the fruit trays and can be easily painted white. Let the white paint dry before using, and you have a nice light weight palette! Use an extra one as a cover. I also like an inverted Rubbermaid oblong container. The lid can be used as a palette, and the container can be used to seal up the paint. ( At home I use a very large Rubbermade oblong container and set my meat tray palette inside of it , mist with water and seal it up until tomorrow!)
The best part about using acrylics is that if it is not going well I can just work on a different section of the painting and let the previous one dry. Then I can come back and change what I didn't like. You can paint over mistakes as many times as you need to with acrylics. But if you are doing it En Plein Air, try not to change it too much, or you will loose the effect of capturing the moment.
The next best part is my that paintings dry before I load them in the car and they do not get smudged or smeared. Smile.