Using white gel pens is an easy way to introduce white back into watercolour sketches. It's definitely more convenient than using masking fluid and for outdoor sketches, you don't have to bring masking fluid along. Click for a zoomed in image
The pens mention in the video tutorial are:
Uni-ball Signo UM-153 with 0.7 line
Uni-Paint oil-based white marker
Pentel 0.8mm
Pentel 1mm
For more of my sketching tips, visit this Youtube playlist and this page