"We have no right to attention unless we know how to command it."
You can't keep what you don't get. So, devote your first sentence to getting the reader's attention. If you get their attention at the beginning, it'll be easier to keep over the course of your work. Attention getting should be one of your main goals as a writer. So, get the reader's attention any way you can. Do whatever it takes to get your readers attention. Remember, in most circumstances, no one will read your work unless they want to. This is the hard world that writers face. Sure, your mom reads everything you write, no matter how boring it is. But, unless you want to write for only your mom, you need to think about getting attention.
Here are some questions to ponder as you strive to get and keep your readers attention:
1) Why should they (the reader) want to read this?
2) What is fascinating/interesting about my topic? How can I communicate this?
3) How can I surprise the reader? 4) Can I tell the reader something they don't know? Something they are not used to hearing?
5) Why is this work important? Note: if you don't find your work important or interesting then -- you can be sure your reader will agree.
Here are some examples of good "attention-gettting" sentences:
Have you ever wondered why clouds are white?
This essay will save your life.
How many seconds does the average person live?