When you want to remind people to do something, you can use the phrase "This is a reminder to ___":

This is a reminder to please fill out your time sheets by end-of-day Thursday.

You can also add the word "polite" to make your announcement sound less bossy:

This is a polite reminder to clean out the break room refrigerator at the end of the day each day. Please do not leave personal items overnight.

This phrase is formal, so you should use it when writing an email or making an announcement to a group of people. When you're talking to someone one-on-one, it's friendlier to say something like:

Hey, don't forget to fill out your time sheet by Thursday.