In order to avoid joins between areas painted in different periods of time, you need to paint the whole surface as quickly as possible. This is a two step process:
Use appliances for cleaning hands from paint stains. You can buy these in DYI shops. They exist in two forms: cloth or paste, and contain solvent that dissolves remainders of the paint.... more »
Clean the paint brushes first. Wipe the paint into old newspaper and/or rinse the paint brushes in white spirit. Thorough rinsing is not necessary though.... more »