Assuming you mean a transmission slip rather than ghost shifting - your small chainring is worn.
Or do you mean it slips of one cog onto a smaller one? Can be painful if it happens when you're out of the saddle - and it can affect the pitch of your voice.
Have you greased the 'innards' of the shift lever? If there's to much lube in there it can mean it doesn't 'grip' properly. I find that just handling the washers etc. with slightly oily fingers is more than enough .
What make levers are they?
Today is the yesterday of tomorrow.