Terorist: There's hard sci-fi and soft sci-fi. Hard sci-fi is more or less an attempt to guess what real life will be like at some point in the future (or present or past, with the whole altered-history genre), extrapolated from what we know and can do right now, with maybe the occasional thing that can't be disproven thrown in (look at The Stars My Destination, one of the best works of "hard science" out there- jaunting's got no modern parallels, but it hasn't been entirely ruled out as a possibility). Soft sci-fi equates the word "science" with "magic", and people who write it can do whatever the hell they like- science fiction doesn't REQUIRE a basic conception of science, it just makes it a better read for those who understand it, in the same sense as a good story set in Singapore will be dismissed (no matter how well-constructed) by the Singaporeans if the author's never been there and based his book on a postcard he got from a giftshop at the local airport. Star Wars SF is so soft that it practically deserves one of those little disturbingly cherubic infants advertising it. That doesn't make it inherently inferior necessarily- personally, I think it's intellectually lazy to take soft SF any more seriously than to enjoy it in whatever book/whatever it was made for, but that's fine. Let it rest.
Sandwich: Er... the multiple-shield system Thunder recommended would be far and awaythe best way of going about that, and would mean something that would be useful for more than just your Star Wars project besides. Beam cannons don't intercept plasma-type weapons (or other beam cannons), and would generate crossfire that'd be hard to explain. The shields could provide "shaped" coverage, would weaken with successive hits, would actually neutralize the target all of the time (I wouldn't trust a beam to hit a Harpoon going at 80mps), would allow for a better impact animation, and wouldn't provide the hard-to-explain phenomenon of fighters 600m away on the opposite side of a missile suddenly exploding. The explosions would probably remain, but there's not much you could do about those unless someone wanted to create special-impact surfaces, so I'd say you should probably learn to live with it.