This would probably need some support from the miner program to sleep between loops.
You can also adjust the priority of the miner process. This will affect a lot in CPU mining, but hardly none with GPU, FPGA or ASIC mining.
UNIX-like: Use commands like nice, renice or top. Big numbers mean more niceness and less CPU.
Windows: Open the task manager and right-click the miner process to access the priority options. Higher priority means more CPU.
Total CPU load will still show 100 % in use, but when multiple threads or processes are running simultaneously at the same CPU core, the miner program will get less CPU than the other processes, thus making the system more responsive but still not wasting any potential mining power by sleeping when not neccessary.
You can also do it another way and give your miner the maximum priority (making Windows hang up). On Unixes this usually means a negative nice number. This makes sure your CPU miner keeps running decently even if, let's say a web page with a lot of flash advertisements, tend to steal your CPU away from mining process. Or keep the miner at normal and lower those other processes abusing the CPU.