As you know, you should include lots of negative match keywords in your keyword lists. You have to think about the terms that will indicate a mismatched prospect type (one who is not probable to buy your product/service). If you can avoid showing your ad on such queries, you can save yourself a nice chunk of wasted advertising dollars.
Recently I came across a huge list of keywords that you can include in your keyword lists as negative match terms. Select the ones that apply to you and save yourself a bundle.
Note: Many of these terms might actually be useful to you and may indicate a higher quality prospect for YOUR products. So check the list carefully before using that.