This is my standard answer to out-of-the box urination problems – you may find the links to be helpful:

First, take her to the vet – urinary infection &/or urinary crystals are the most common cause for cats to start going outside of their boxes. Tell the vet what’s going on, and ask for a urinalysis. If she’s ill, she’s in pain and can’t help going out of the box occasionally – behavioral remedies will be useless until she’s treated for the urinary health problems. Cats are good at hiding illness and pain (it’s a survival tactic), so often the only clue you’ll get that your cat may have urinary health problems is out-of-the-box urination.

If she checks out OK at the vet, it’s possible that something about the litterbox itself, the litter, the box location, etc. is bothering her. See these links for suggestions:
http://www.catinfo.org/litterbox.htm
http://www.vet.cornell.edu/fhc/brochures/Housesoiling.html
Those links don’t mention it, but I highly recommend trying Cat Attract litter – it’s available in most pet supply stores.

Stress can cause inappropriate urination – cats can be stressed sometimes by things that we don’t even notice, until we know what to look for. This article explains stress in cats & how to address it:
http://cats.about.com/cs/healthissues/a/stress.htm

A Feliway Comfort Zone diffuser can help a great deal with stress & litterbox issues: http://www.petcomfortzone.com/czcats.htm

This link has extensive "out-of-the-box" resources: http://www.geocities.com/Heartland/Pointe/9352/litterboxhelp.html

This link has excellent urine spot locating and cleanup info: http://www.thecatsite.com/Care/48/Combat-Cat-Urine.html
It’s a 2-page article (the link to the second page at the bottom may be a little difficult to spot).

Hope this helps!
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If the vet finds nothing wrong, there are many things you can try. You may need to provide another litter box, use a different type of litter, remove the hood (if it’s a covered box), or clean the litter more often.

You might need to cover the furniture with plastic (a shower curtain liner, table cloth, or painter’s tarp) when you can’t supervise the cat.

Comfort Zone helps with marking and anxiety-related problems. It uses a pheromone to make the cat more comfortable.

Use an enzymatic cleaner to clean up any urine. This is not a repellant; it removes all urine so there is nothing to cause any odor. Most products that are designed to repel are not very effective, in my experience.

If none of this works, contact a behaviorist. A lot more info is needed to figure out why your cat is doing this. I just listed the most common solutions.
References :