It's a common problem, even for those with whom writing is their profession.
What kind of environment do you write in? Is it quiet, free from distractions? What do you use to write? If you are using a computer, close down all applications except the one you use to write, including instant messenger apps, email, browser, etc. This will help if you are easily distracted (you might not even realise you are easily distracted).
What time of the day do you set aside for writing? Everyone is different, but generally I find my brain is better at this sort of thing shortly after waking and having breakfast. It gets harder in the evening, and anything I write late in the evening needs proofing carefully the next day!
Another good way around this kind of mental block when writing (its not quite writers block, that is deeper and usually not a temporary thing), I find, is exercise. Get some blood (and therefore oxygen) to the brain, and you'd be amazed at how many ideas start coming through, all at once. I've often wished I had a pen and paper when out running so I don't forget any of them when I get back to my desk.
We're all different, and finding the right conditions for you to write in may take some time. Some people are happy writing and working in Starbucks, whereas that drives me crazy - I need solitude, most definitely. Perhaps try finding a quiet spot in a park with just a pen and paper? Nature is inspirational and the walk to find a decent spot will get the blood going and the creative juices flowing.
Best of luck! Let us know how you get on and share your tips with us
