One way is to remember is to break it down into the parts that make up the derivation. It is based on the word "effektiv", "-itet" is a common way to make nouns out of (especially Latin-based) words.
In Dutch we make a difference between effectiviteit (how big is the effect?) and efficiency (is, regarding to the input, the effect quite big?). Is the Swedish "Effektivitet" used for both?
Mmh, it is striking that effektivitet is en 'en' word and e.g. universitet is an 'ett' word, so apparently words with -et ending can be neuter or non-neuter.