I once wanted to train up to run a half marathon. What I did was to run two, sometimes three, times a week. But to build up my stamina, I pushed my limit a little more every week by increasing my distance by about half a kilometre. After six months, I had increased my distance from 2km to 13km.
Practising music is alot like that. More often than not, I like to pick up the guitar, enjoy a few riffs that I've played a thousand times before, jam to a backing track, or play with a cd. These things, however, don't really push me to my musical limits.
To be a better musician, you need to practise with a deliberate intention to improve. This means two things i) you need to repeat difficult riffs alot to master them; ii) you need to choose riffs that are just beyond your ability to play to stretch your limits. I find that a metronome is quite essential in helping you stretch yourself, as you can very gradually increase your tempo in a structured way (i've provided a free metronome on the right sidebar).
However, it also means that practices are less enjoyable, its like all the panting and aches I got while training to run further. But you need to look beyond that, and think about the pleasure you get when you finally master that difficult passage. Or in the words of Optimus Prime, "no sacrifice, no victory!".
For more, do check out this article I chanced upon in Fortune - "Why Talent is Overrated".