Answer: The best way to clean travertine is with a bucket of water, a mild soap, and a soft-medium brush. duPont StoneTech has a spray cleaner that works well. If you have tough stains that will not come out, you can use a poultice. Miracle Sealants has a great one (Click to buy: http://amzn.to/1gBwqw3).
With regards to the sealer: DuPont™ StoneTech® (Click to buy: http://amzn.to/1ouZqs2) has some of the best sealers on the market. Just remember to test it on a small tile to make sure it is the look you want.