The demonstratives is, ea, id are definitely tricky, but once you get a few general rules down they should be a breeze. Here's how I keep them straight.
- Across all genders, the dative singular is the same--eÄ«.
- Across all genders, the genitive singular is the same--eius
- The neuter follows the "neuter rule", ie., the nominative and accusative are always the same, in both the singular and the plural.
- The neuter plural (ea) is always the same as the feminine singular (ea)
Other than those things, you should also pay attention to the thematic vowels. In the masculine, this is an o, though a u shows up in the accusative singular eum. In the feminine, the thematic vowel is an a.
I'm not really sure how clear this all was, but hopefully it helps a bit. There aren't really any mnemonics to help memorize these things, but it is helpful to notice and recognize certain patterns.
Fortuna bona tibi!