Lap marks occur when you attempt to paint over paint that has already started drying; to avoid lap marks, you need to merge areas of paint while they are both still wet. It's hard to control how fast the paint will dry when you work outside, since you're at the mercy of the weather, but following a strategy and using certain techniques will help you avoid lap marks. If you are painting vinyl siding, the techniques you use for avoiding lap marks are similar to those for any other material, but vinyl does require a few special considerations.
Stir your paint well, and thin if necessary to make it dry more slowly. Additives that can be stirred into the paint for the same effect are also available. Use only latex or acrylic paints for vinyl siding; the siding expands and contracts quite a bit, and water-based paints will withstand this. Latex and acrylic paints can be thinned with water. In addition, higher-quality paints work better for avoiding lap marks than lower quality paints.
Paint in optimal weather to avoid having the paint dry too quickly. Temperatures should be between 50 and 85 degrees Fahrenheit -- never paint when it's above 90 degrees. Avoid direct sunlight by painting the west side of the house in the morning and the east side in the afternoon. Humidity will help slow drying time, but rain will ruin your work, so check the forecast before beginning.
Apply paint to only a few strips of siding at a time. Working in a small area makes it easier for you to keep the edges of the paint wet. While it's tempting to move the ladder as little as possible, moving along only a few strips of siding will help prevent lap marks. Start from the top of your house and work your way down. Using scaffolding rather than a ladder can help with this project.
Paint toward the unpainted area, then back, rather than the other way around.