How can family visit me in the UK while I live here on spouse visa?

Hello! I am a America citizen living in the Uk on a spousal visa have been here for a year now. I was wondering if you knew what had to be done in order for my mother to come over and visit and stay with us for a few months as I know that tourists can stay up to 3 months without a visa and this is what she was wanting to do? I just would like to know if anything special needs to be said to immigration officers upon entering the UK or if saying she has come to visit friends and family for a couple months would be OK to say? Please help!?
Thank you!

Nothing special required, it can be a good idea to have something that proves ties to the US, such as a letter from employer, rent/mortgage, oh and a return flight.

Unless it's changed when I wasn't looking, visitors can stay in the UK for up to 6 months rather than 3.  I understand that it isn't a given that a passport will be stamped for 6 months rather than the length of time till a return ticket.

I agree with what Shandy said as far as what you need to show the IO, but if your Mom is worried she can apply ahead of time for a visitor's visa.  Not required, but it does give peace of mind.