Give your students a place to keep track of their homework. This form has a place for 20 days of homework. I usually staple it into their workbook or notebook and check that they have filled out the day's homework before they leave class. That way the old, "I didn't know" trick doesn't work.
This is the same idea as the one above, but with spaces for 14 days and a slightly different design. It's a word document, so teachers can change it around a bit to suit their needs.