I HAVEN'T GOT TIME!! Is the usual answer. But I know they have the time, so here are a few tips that I give to my students.

I hope they work for you.

1. I always give students their own practice sheet to fill in their own times. Practice is a routine, therefore this sheet will help them realize how much, or how little they work, and on what days etc. Just like a business analyzes information to make it more productive so must every drummer.
2. I ask every student to carry a small practice pad (big enough to fit into a bag or jacket), and sticks with them. You never know where you might be, with little to do. So work on those sticking patterns, or rudiments! Use the dead times in the day, you could increase the amount of time by an extra 10-20 minutes per day!
3. Try taking a piece of music/sight reading, and work on your ability to understand notation. You could try singing it in your head, or tapping on your legs or practice pad.
4. The weaker hand! Most people have a weaker hand, so you should be trying to get it equal to the stronger one. Try carrying a squash ball in your pocket. You could work on squeezing exercises, anytime you like. This will benefit your playing lots, think about a boxer trying to box with only one hand! You get the picture. I see my students playing fills or rhythm patterns, and some sound so uneven! So try and get those hands working together, everything should sound musical.
5. Every three months try writing out your name on a piece of paper(using your weaker hand). Any improvement? Keep a record of this to see how you go.
6. When you practise make certain that you don't have any distractions (turn that mobile phone off!). Sit in a warm environment, NOT in a cold bedroom with the heating off!
7. Work slowly and go for accuracy rather than speed. Speed always comes quicker when you really know an exercise.
8. (WALK THEN RUN). Also if you can't grasp an exercise in one go, chop it up into two or maybe three sections. Then when you have practised each section put them back together. (Remember you wouldn't try and eat your lunch in one mouthful!!).
9. Stay focused, don't just practise the same thing or try and play everything your favorite drummer plays! Work on all aspects of your playing, reading, technique, co-ordination etc.
10. If possible work with a metronome (there are many small portable models, good for pad work). Also playing with a drum machine great for getting the groove! Hook up with the bass player, and play along, as you both form the foundation for the music.