Since the basic Marines feature virtually no dinosaur-related paraphernalia, in order to tie the squads in with the army theme I've painted dinosaur icons on the Sergeant banners. Freehand is a relatively simple way of displaying themes, though the technique can seem a bit daunting to beginners. Keeping it simple is the key. As you can see I've only used white for these icons. Keep the paint thin so you can easily correct mistakes. Starting out with basic shapes like lines and circles, and sketching out the rough shape from there will help a lot when painting monsters or other complicated images. I'll put up a full tutorial on painting the basic Space Marine symbols (Tactical, Assault, Elites etc.) later.
Apart from the slightly altered scheme (black instead of red shoulder pads), the Veterans are distinguished from the regular troops by decorative horns made from Tyranid claws (in particular, Genestealer talons). Not only does this give them a feral look (in-keeping with the chapter's background), it also furthers the whole dino/dragon theme. Small additions like these, when applied to a whole squad, can be very effective.
In order to set the Honour Guard apart from the regular Veterans, slightly larger horns were added to their armour, as well as Night Lord wings to their helmets. This latter conversion was based on a sketch a member of the Warseer community made for me a few years ago. This is probably the best thing you can do when you're stuck for new ideas to implement a theme: ask around!
These were the first models of the entire army to be converted in any way with the dinosaur theme in mind. I used Genestealer talons as tusks, while Scything Talons were added to the Sergeant's back to set him apart from his squad.
So let's take a look at theming vehicles next.