I take a slightly different view. I think our judgement of creative thoughts and ideas changes depending on what we're doing.
If you have an idea while you're driving, you're probably going to cut it quite short with the new idea: how am I going to remember this idea so I can write a song?
If you have the same idea at home while sitting down with your guitar/ notepad/ whatever, you might sit with it for a while and then decide it's not worth pursuing. Or you might play around with it, and not be able to take it any further. The crucial difference is you're better placed to judge its quality.
The best remedy for this in my opinion is to create space in your routine for writing. Of course, some days you'll come up with nothing or just awful rubbish, but if you persist, you'll begin to open the channels, and good stuff will come. Having said that, it is also worth carrying a notepad around though