Examine the roots
Resist the temptation to simply grab your plant out of the old pot and put it directly into the new one. Give some attention to every single plant. Once you take it out of the soil, gently shake it, so that the roots lose their previous pot-like shape. Carefully remove all the rotten parts and closely look if there are any parasites or infection.
The perfect size for a new pot is usually double the size of the old one. If you take a pot that is too big (for instance, because you want the plant to “feel free” or because you do not want to stress the plant by transplanting it more than once), it will only give you an extra headache. First, you will waste too much water on the “empty” soil. Second, since the roots will not be able to absorb all the water in the pot, you will risk ending up with rotten roots. Not to mention that the more space your garden takes, the more electricity you waste.