Quoted:
I've replaced the sights on 2 1911s and two USPs.  Get a brass drift from the hardware store and hammer it against the sight.  Brass is softer that steel so it won't score your slide and ruin the finish if it slips a bit.  Use steel of some sort to push the sight out and you can hurt your pistol's appearance pretty fast.  I also like to cover any part of the slide near the sights with blue painter's tape.  It won't prevent a heavy blow from scratching the finish but it does provide some protection at least.  On my Kimbers it took A LOT of force to start the sight on its way out and it gets a bit easier after that, but not much.  Also, the cut for the sights isn't always straight across and symmetrical.  Make sure you know which side to push the sight out of.  That info should come with the sights you buy, but if not, call the sight or pistol mfr before you hurt the weapon.