Trying to improve my practice

It's not that i don't meditate. I'm doing it even regular. But not as long (~15-20 mins) and regular (3-4 times week) as it should be. Working the whole day (plus a pretty long way home) its really hard (for me) to sit down, relax and take the time you need to watch your breath and cultivate your awareness. Of course, it always depends on your current state of mind. One day i have to start three times to be able to enter some kind of samadhi, the other day my practice appears to be good from the moment i sit down and find the correct posture.
One thing i like about meditation is the possibility to train your abilities. I sometimes compare it with strumming my old guitar or playing soccer. Even if you haven't done it in a long time, you don't forget everything. Like a sport you maybe need a couple of days to be back on top of the game, but you never have to start from the beginning again. At least i feel it that way.