What is the best product for sealing tile grout on floors?

I have scrubbed the grout in my tile floors on my hands and knees and then sealed it, several times over the last 4 years. I never seems to stay cleaned for long. I have a special needs dog and sometimes she pees on my tile and I can never get the grout clean, even with my steam mop. I have tile throughout my entire house but the kitchen is the worst to keep clean.