Something I share with my students, and very similar to Dadfad's advice, is to strum with your thumb while listening to songs you want to learn. This is a good way to get some practise when you can't have your guitar in hand. I drive down the road, tapping my fingers on the steering wheel, or strumming my leg with my thumb. They both go hand in hand with Dadfad's advice.
Also, set up a diagram similar to the one below, and use it to create and practise new strum patterns. You can copy and paste this into Notepad to line it up, or add new patterns.
CODE
********************************************************************
Strum Patterns
D = Down Stroke
U = Up Stroke
4 --------------------------------
- 1 & 2 & 3 & 4 & 1 & 2 & 3 & 4 &
4 --------------------------------
D D D D D D D D
D D U D D U D D U D D U
D D U D U D U D D U D U D U
D D U U D U D D U U D U
D (D) D U D U D (D) D U D U
D U D U U D U D U D U U D U
3 ------------------------
- 1 & 2 & 3 & 1 & 2 & 3 &
4 ------------------------
D D D D D D
D D U D U D D U D U
D U D D D U D D
********************************************************************