Making cake icing look professionally smooth
Professionals ice on a turntable. Turntables for home use are affordable, and make icing much easier. 

Your spatula should reach to about the centre of the cake when held steady and comfortable. As @rfusca suggests, heating it for buttercream is a good idea. You can also wet it for other icings, to make it glide smoothly. 

For the icing process itself, start with the top of the cake. Apply a generous amount of icing; you will be thinning it out with the spatula, not smearing it right and left to get an even thickness. Hold the spatula to an angle to the cake, tip at the centre, and rotate the turntable. Let excess icing drop to the side of the cake. When you have done the top, smear icing thick on the side, hold the spatula vertically, the edge at 45° to the cake side surface, and again turn the turntable. When you have finished, you will have raised icing on the edge of the cake. To finish the cake, smooth these edges. 

To do this, you need enough icing. "The professional chef" recommends 340 g (12 oz) for a 20 cm (8 in) cake and 454 g (1 lb) for a 25 cm (10 in) cake. 

This all assumes buttercream icing or other spreadable icings such as ones based on whipped cream or creme fraiche. It is somewhat tougher for semi-liquid glazes such as ganache.