Chain slipping in cassette when under load, could a different type of cassette work better? This is my second modern bike and I got the same problem each time. The chain slips when under load (usually when I'm standing, but I can slip starting at a light and still sitting too). At first I had an aluminum cassette and the dents broke quickly. I replaced that old cassette (that actually came off as it completely broke) with a steel cassette.

The steel cassette has been working well in the sense that the dents are still all looking good and it has not come into pieces yet, however, the chain still slips.

In both cases, I had 9-gear cassettes when I use the smallest gear 99% of the time (San Francisco type of hills still require me to change gear, otherwise, I rarely change anything, so I could assume that the small gear of the cassette gets worn down quickly...)

Would it be possible to have a cassette with longer dents to avoid the slipping? My old steel frame bikes had cassettes with really long dents, in comparison. Could a find similar cassettes that are compatible with my new bike? (I have a specialized Roubaix now)
The chain will typically slip under the following conditions:
 1. the chain is worn out (this can be measured with a cheap tool,
    or by your LBS).
 2. the cassette is worn out 
 3. one or more of the chainrings (the cogs on the crankset) is worn out 
 4. the chain and/or cassette/cogs/pulley wheel are dirty and full of oil/road-grit/gunk.

These can occur simultaneously - a dirty chain causes rapid chain wear, a worn out chain causes the cassette to wear out.  If you have been riding with a slipping chain for long, you are very likely to have worn out both the chain and the cassette and will have to replace both.