I use a small xacto blade, depending on the shape of the part I either carve down forward like RustyFord said, or scrape backward like someone else did. (I dont scrape toward detail parts, but away from them, on long stretches use very light pressure and carve it down)
I also have used 22 and 18 gauge hypodermic needles in the past. (I spent many long years as a Respiratory Therapist and they were freely available) The tip of the needle is precision laser cut and sharply bevelled so they slice things easily and because they are light and so small it is hard to bear down with them too much pressure and overdo it. Just turn the edge of the bevel at about a 90 deg angle to the piece and scrape. Practice on scrap first to get used to how much pressure and what angle to hold it at. It takes a while to whittle things down like this but you only touch the mold line and it's hard to carve out gouges in the plastic.
I'd recommend an 22 to 18 gauge (smaller the number the bigger the needle) for 1/48 up and 25 to 20 for 1/72.
Check with the pharmacist and if they won't sell them to you a livestock feed store should.
just keep your fingers clear of it when slicing
and be sure to cap them when not in use... I've been bit one too many times by one of them for my pleasure...