Use something water soluble that is easy to wipe off, and then practice. Wipe off if not satisfied, try again, and repeat until it looks good. Wiping along a line will remove most or all paint, while wiping across will leave more paint in the crease.
The most common mistake is to overdo it. Look at photos of real cars - they have very thin panel lines. The moulded panel lines on 1/32 car models are usually much too wide. If you fill them with black ink it looks awful. Experimenting with shades of dark grey instead of black, and then wiping off most the wash can often give a better effect.
If the panel lines are well moulded in the first place you don't really need a to fill with much paint. The GP-Miniatures resin bodies are extremely good - the panel lines are deep but narrow. Graham scribes the panel lines on the master mould with the tip of a very thin blade.
Still, when you paint a multi-colour scheme and add decals the panel lines may get partially covered. A dark wash will help to enhance and even out the lines.
The DBR1 below was assembled from a GP-Miniatures body. The panel lines were invisible where they passed the white roundels - a dark wash brought them back.