If any character from SaMoon falls in love with someone from the other 
series (besides Makoto): 2 drinks
If it's Makoto: 3 drinks

Nah, that's only 1 drink.  Far too common.

If it's one-sided: 5 drinks
If sex is involved: 3 drinks.

2 drinks.  See above. :-)

If most of the Ranma 1/2 and Sailor Moon casts decide to get together and 
have a huge honkin' orgy: Go to the store.  You're going to need more.

Paging Mr. Biles...

Authors: 
1 Drink
Chris Davies
    And it's not a darkfic, 4 drinks
John Biles
    and it's not named after a set of musical works by Grieg or done 
    with an Arthurian setting, 5 drinks. 

Andy Combs
Mark Latus
Frank Barr
    and it's not in their big honkin' continuity, 7 drinks

--Sean Gaffney
--and leave us not forget authorfics..."I was a Sailor Senshi!!"