The traditional combination of 53 and 39 tooth chainrings with a 12-23 tooth cassette seems to work well for professionals, but such gearing is not ideal for all riders.
Compact cranks are finally coming to the forefront as a viable option for riders of all abilities to consider. Anyone living in mountainous or hilly terrain should give serious thought to the benefits of a compact crankset. Rather than using the standard 130mm Bolt Circle Diameter (BCD), compact cranks have a smaller 110mm BCD that allows for smaller chainrings. Compact cranks commonly have a 50 tooth large ring mated with a 34 or 36 tooth inner ring. These smaller rings allow for lower gearing and a higher climbing cadence. With this setup you not only have an additional lower gear, but also a higher gear (when compared to a traditional set up of a 53/39 crankset with a 12-25 cassette). This wider effective gear range is accomplished with fewer duplicate gears.
Standard cassettes are usually a 12-23 or 12-25. Switching to a larger cassette could be the most cost effective way to achieve a suitable climbing cadence. A 12-25 or 12-27 cassette might make all the difference you need in achieving optimal gearing that allows for an efficient climbing cadence.
So what gearing option is most suitable for your riding? There are a couple of important considerations. First, are most of your rides on flat roads, in the mountains, or somewhere in between? Second, what is your current fitness level? A Cat I racer will be able to use standard gears with an optimal cadence while riding up most climbs. In this situation there is little or no need for lower gearing. If you are very fit or ride mostly flat to gently rolling terrain, a standard double crankset with a standard rear cassette will likely work just fine.
Riders and racers living in very hilly or mountainous areas, serious and novice alike, prefer the versatility of a compact crankset coupled with an 11-23 cassette.