Once you've decided what your thesis is going to be, you must be able to frame it in a manner that provides an effective entry into your work. No matter how great your argument is, it will not do much good if no one is enticed into reading it. The two most important functions of your introduction are to serve as a grabber (a stylish, creative lead-up to what you're trying to say) and as justification (an explanation of why your argument is even important in the first place).
DON'T summarize - Though it might seem easy to preface your thesis with only a synopsis of the texts you're writing about, this is a particularly dull way to begin a paper.
DON'T keep reiterating your thesis - Your thesis should appear in your intro as the culmination of the previous thoughts, not just something you mention and then keep restating to fill up a paragraph.
DO ask yourself questions - Why is your thesis relevant? How is its being proven important to the understanding of either text or fact? By linking your argument to a larger issue, you will give your argument both universality and interest.
DO be creative - Think about what aspect of your topic you find the most interesting, and figure out why. Use this to make it interesting to your reader.
The quotation - Find a quote from one of your sources or, even better, from elsewhere that seems to get at the problem you're dealing with. State it at the beginning of your intro and discuss how it relates to what you're trying to prove.
The question - Throw out a broad question of universal interest, and demonstrate how a possible answer can be related to your thesis (Example: "What do women want? It's a question that's plagued mankind since the dawn of history...the works of Emily Dickinson and Sylvia Plath yield two different paradigms of feminine self-realization").
The anecdote - This works particularly well for a historical essay, and even better if you have some ability at creative writing. Pick a specific incident that represents the underlying conflict of your piece, and briefly narrate it like a story. Explain afterwards how the instance reflects a problem you're attempting to solve.