Try C-x C-+ and C-x C--; that is, Control–x Control–Minus/Control–Plus.
After one combination (C-x C-+ or C-x C--), successives + or - increase or decrease the text scale without typing C-x C- again.
Addition by sawa
I looked up the function that was assigned to the keys mentioned, and found out that they are text-scale-increase and text-scale-decrease. I added the following to my configuration file so that I can do Ctrl+Scroll to zoom in/out. It is useful.
(global-set-key [C-mouse-4] 'text-scale-increase)
(global-set-key [C-mouse-5] 'text-scale-decrease)