You need to stimulate the right side of your brain more. Try listening to music or watching something on TV while writing. If you're not on a deadline and just writing for personal reasons, then never force it.
You'll hear "professional" writers constantly bragging about their ability to write on bad days. But most of what they churn out is complete garbage. Another thing you can do: work on your craft in your downtime (e.g., structure, grammar, and style). I adhere to a combination of the AP Stylebook and The Chicago Manuel of Style. Sometimes, I look to Bill Walsh's Lapsing Into a Comma for guidance (your editor's preferences really become yours).
If your left side is just dominating, then feed it facts. I am a fan of writing on the go, and it's good practice to write descriptively about the physical places surrounding you. If I really like the end result, I'll put it into one of my stories; this saves me a lot of time.