Three elements of an effective introduction

The introduction to the essay must perform three functions. It can be said to be effective or ineffective to the degree that it carries these out. 

First and foremost, the introduction must captivate the reader. If the reader doesn't feel like sitting up during the opening of the essay, slogging through the rest of the essay will be pure drudgery for them–and worse, it will be at your expense. Use one of the tried and tested techniques to capture the attention of the audience. Start your essay with a rhetorical question, a story, a quotation or a striking fact relevant to the forthcoming thesis, or at the very least, related to the  topic. 

But simply using one of these techniques at the outset does not guarantee an effective opening. The attention-grabber must transition steadily to the thesis. The bridge between the catchy opening and the central claim must be short as well as strong. Right after employing one of these techniques, use a phrase or a sentence pointing straight in the direction of the thesis. Don't explain the attention-getter. Don't comment on it.

The third element of an effective introduction is the essay preview. Next to the thesis itself, the reader would feel very pleased to see a snapshot of the unfolding essay. It can be done by expanding on the thesis in a few sentences or better, by laying out the skeleton of the essay. 

To recap, the attention-grabber should give a heads up to the thesis it wants to spotlight before the introduction is tucked in neatly with the help of an outline of the main points.