I'd suggest a thorough understanding of the plot and characterization associated with either series. Often the heaviest flaws in crossover fanfiction reflect a lack of thorough understanding and consideration of the particularities of the involved series. This is especially important when combining series with little similarities (for example a crossover between My Little Pony and Ranma 1/2. Very few similarities there). A good crossover is very difficult if the series have little common ground to work with.
Often the best crossovers integrate the two series into a single world without strict reference to there ever having been a separation to begin with. This is pretty difficult to accomplish and I think is best achieved through experience.
Also, the most popular crossovers tend to have a standard set of clichés, which are best avoided or handled carefully. Like... a popular cliché with any Sailor Moon crossover is for *insert character name here* to discover he/she is a long-lost Sailor Senshi/sibling from the Silver Millennium.
(same with Dragonball Z. Oh my god! You mean in a past life Usagi was the heir to the throne of the Planet Vegeta? She somehow escaped the devastation of the planet exploding, because she was on a mission elsewhere at the time 0.0
).
Or...
Oh my god, no one loves me anymore. I'm such a screw up, so the Silver Crystal needs to take me to a world where I am loved and appreciated. Oh, hello, Duo from Gundam Wing. I've never heard such deep words of passion and love and appreciation and devotion! Obviously you are my true soul mate, not that Prince oh-what-is-his-name. You all love me more than my sailor senshi ever could.