Removing the spent blooms will certainly help. You should also make sure the plants are receiving plenty of sunshine and are kept evenly moist but not sopping wet.
A regular fertilization program with a water-soluble fertilizer for flowering plants and/or topdressing with compost every few weeks should also help.
Finally, make sure the plants are planted in pots suited to their size and that they have been pinched back early in the season to encourage branching and bushiness. This probably does the most to enhance multiple blooms at a time; another possible consideration would be to select a different named variety of geranium. In some cases the patented varieties do outperform the seedlings grown for mass bedding use.