1. You can try using bi-grams or even tri-grams to make you word list a little more precise.

2. Create a validation by manually identifying each review as positive or negative. Each time you modify your algorithm run it through your validation set and note the results in a spreadsheet. If you don't do that, you'll never know if and how you've improved the results. The bigger the validation set the better. Similarly, you can use part of your validation set as a training set into a classifier.

3. Find a scale that works to bias your score. For example, I would try to bias your negative score using a log scale. The fewer negative words you have the more they are worth, the more you have the less they are worth.