I've reccently discovered a load of monsters I created for 3.x D&D and would love to convert them to 4E.
Do any guides exist for this? Or could someone give me the most important points to remember when re-creating a monster into 4e?
Take a page from Robert J. Schwalb.
He's systematically updating monsters from the epic level handbook to 4e. The essence is to ignore the mechanics, and preserve a subset of the unique iconic attacks.
Generally you want a few iconic powers, rather than a lot of fiddly powers. The best way to learn is through memisis of Schwalb.
Focus on rebuilding a monster using 4e rules, not converting it. Most monsters in 3E are over-engineered to an extent, in that they have many more things in their statblock than players will ever see during the game. The basic rules for creating 4e monsters can be found in the DMG.
Decide what the one or two defining features of a monster are, and create powers that reflect that. E.g. A phase spider can teleport. Make sure it has a utility power that allows her to teleport, and maybe create an attack that includes teleportation as 'for free'.
Don't worry if you don't capture every feature, or if the monsters seem 'easy' compared to their 3.5 counterpart - it's ok that a 4e medusa only gets to petrify (save ends).
What I usually do is to also create an encounter location for the monster that works well with 4e rules - as 4e puts a lot of emphasis on placement during the battle, think about how the defining monster feature mentioned above interacts with the environment.
The phase spider is best utilized in a place where spider webs hinder movement, and the medusa is more memorable and dangerous if there is unstable ground or dungeon traps that move petrified player characters into less optimal positions.