· Keep an eye out for interesting real life characters. My driving instructor seemed overly critical of my inability to drive, given that that was the reason I was employing him, so I wrote a sketch about an instructor who berates his pupils for being non-driving idiots.
· Don't start with a catchphrase. It will seem forced and probably end up with you creating a one joke persona. Create the character, write some sketches and a catchphrase might present itself. Look at Al Murray the Pub Landlord. It's a multi-layered persona and the catchphrases "I was never confused", "rules is rules" and "glass of white wine for the lady" come out of the character rather than vice versa.
· Starting with a simple premise and exploring the consequences can be better than trying to conceive something outlandish. Monty Python's dead parrot sketch begins with the premise of a pet shop owner selling a customer a deceased bird. The genius is in the execution.