Every photog worries about model releases and getting signatures. With the mobile revolution upon us I'd recommend something like this product.
I backed their Kickstarter many moons ago and use the original version of the hardware to this day for signature capture. Far better and less goofy than a finger scribble.
@Phrodo_00 I've not use the Adonit offerings and my stylus is an 'old' model w/ a slightly larger tip than the current one.
I've had good success over the years with it. However, on 'small' screens under 5" it can be a little fiddly.
I also noticed that depending on the app the resolution / response can be tricky but certainly a huge improvement over a finger.
The one thing I noticed that caused a lot of skips was moving it across the screen too fast. Seems to be app specific though.