1. Don't wait until you feel like it!
Waiting for the motivation to practice is a surefire way to have an unproductive day. You simply can't rely on your emotions to ever give you the time of day, so don't even try! Practicing music should be considered as "studying" and if you like to study then you know that it doesn't always feel like the most exciting activity in the world, but you do it because it's important.
2. Practice music with others
You'll notice that when someone else is there, or if you have some music to listen to, it will be a lot easier for you to practice music. Who knows why? Maybe you feel like you owe it to the other musician or maybe you don't want them watching while your practicing... Whatever the reason may be, having someone there is usually an awesome motivator.
Having an accountability partner is a wonderful idea if you want to get something done. It could be someone who you email your practice session logs, or maybe just have them give you a call every day. When having accountability with someone else will really help push you forward because at the end of the day you don't want them knowing that you broke your promise to them.