Here are the not-so-secrets of the trade.
Moleskine Sketchbooks I use the large size. A Moleskine watercolour book is lovely too if you like the long landscape format. And watercolour in my experience does not work with 'standard' Moleskine paper.
Crayons Caran d'arch Neo Colour II and smugging and blending them on the page is great with wet finger-tips.
Pens - Anything by Pilot especially the G2 and G4 (extra fine points) ranges are wonderful. Uniball pens also excellent.
Watercolours - my watercolour box (not seen here) is 30 years old now, It was bought for me by my first boss in an ad agency! Sadly the colours need replacing every year or so - I use Windsor and Newton half-pan size watercolours go for 'artist quality' not 'student quality'
All this kit can be easily packed away in a small bag (oh! don't forget a small water bottle and a glue stick for appending bits and pieces and tickets etc to the page) and you are all set!