During my time in Prague last month I went on guided walks with two walking tour companies. The day after I arrived in Prague I went on a guided walk with The Naked Tour Guide. The Naked Tour Guide specializes in small group guided walks (no more than 8 people). I went on the Explore Prague Historical Tour, and it was just me and a couple. And with our guide Marcus we were only a total of 4 people. Marcus was great, not only did he show us the sights on the walking tour, he also brought us along various side streets and lanes, bypassing the large crowds. At one point we stopped at a local cafe for a snack, and while there Marcus gave each of us a free (and excellent) map of Prague. And the tour ended at the Letna Beer Garden, where the 4 of us had a long and great conversation over excellent Czech beer.
You can find out about Marcus and the other tour guides at The Naked Tour Guide at http://www.nakedtourguideprague.com/
I also went on guided walks with PragueWalker, the walking tour company recommended by Rick Steves. One guide at PragueWalker who I highly recommend is Tereza. One day I went on a 4 hour Castle District/Prague Castle/Little Quarter walk with her, and on another day I went on a Communism and Velvet Revolution walk with her. For both walks Tereza was absolutely amazing, not only did she point out the sights and explain all the facts in detail, she did so in a very engaging and personal manner. And Tereza also has a great sense of humor, which makes going on guided walks with her an absolute joy. Going on the guided walks with Tereza was just like having a local friend show you around Prague, I absolutely enjoyed having Tereza show me around the various sights and places on the two walks I had with her.
You can find out about PragueWalker at http://www.praguewalker.com/, though as of now Tereza's not listed on the website's list of guides (as are a few other PragueWalker guides). And if you do choose to hire a local guide from PragueWalker I've heard and read reviews that the other PragueWalker guides are great, though I would strongly suggest hiring Tereza if you can, she's above and beyond excellent, above and beyond amazing.