It's all about PEC error.
Return the rudder to central fairly quickly from a sideslip, and watch what the ASI does. If it rises by 5 kt, then your ASI is under-reading in the slip by roughly that amount. Next time you want to sideslip at 50kt for example, fly at 50kt, enter the sideslip and hold an indicated 45 kt. And vice versa if it over-reads.
That's what they teach you at ETPS, anyway. But like Genghis says, if you keep the pitch attitude the same and don't let the IAS decrease once you're slipping you shouldn't go far wrong.