There may not be any absolute rules, but there are some general rules based on syllables which will work for all but a few exceptions.
Rather than typing them out, I'll let you do the research. You can search for 'adjective comparative and superlative rules', and find them. Post back if you have problems locating those syllable-based rules.
Of course there are a few irregular adjectives which don't follow the rules, so you'll need to memorize them.