Being relatively new at this myself (just comming up on a year now), I'd say that research is your best tool. Get yourself several good books, use the internet and ask questions. Then make your stocking list, and stick with it. The pair of clowns is a great start.
Dottybacks, for instance, can be very terratorial and/or agressive (please note I said "can be" - not "are" - don't want to get in trouble for generalizing an entire group of fishes). Anyway, one of the first fish we purchased was a bi-color pseudochromis/dottyback (in our defense, as newbies, 10 months ago, the bi-color looked an awful lot like a royal gramma - besides the salesperson at the LFS said it was "peaceful" and would be fine in a community aquarium). Boy was that a huge mistake! That little 1 1/2" fish terrorized the entire 90 gallon tank - so back he went to the LFS. I swear that little terror glared at me when they released it into the tank at the LFS!!!
The same salesperson also sold us a green serpant star (Whoops!), insisting it was simply a color variation of a brittle star, totally benine, reef safe and would not be a danger to fish or other inverts (can you say "missing yellow clown goby" - twice?). We found him a new home as well...
We have also fallen victum to seeing something we couldn't live without (without knowing enough about it), only to realize later that it wasn't a smart decision. PJ, our 6-line wrasse is a good example. Bought her on a trip to a LFS, when we actually went for something else. She was so cute... LOL She became a real pest, unfortunately - annoying all the other fish. We recently look her to a LFS, so she could find a new home in a more appropriate environment.
One surprize, however, is our beautiful Copperband butterfly fish. We bought "Dot" to clean out all the aiptasia on our live rock - which she has certainly done. Had read how difficult they can be to keep alive and two LFSs had told me they rarely thrive, even with expert care (I should add that they also recommended not to purchase one - but obviously, that didn't stop me - saltwater probably not the best hobby for someone like me that is stuborn as a mule!)... But we've had her for 8 months, and she's doing fantastic.
My best advice, is take your time, check compatibility, and talk with others that have that particular fish/invert/coral in their tanks. If someone had given us that advice 10 months ago, it certainly would have saved us some headaches, money and stress!
Best of luck and enjoy stocking your tank!