Stay in the slow lane, unless you are coming up to a slip road, then move into the middle to let people in. Then move back. Technically they should match your speed and enter the motorway without you moving or slowing down, but that pretty much never happens. They'll just barge in without looking sometimes, so be aware.
Don't sit in the middle lane the whole time, especially if the slow lane is empty. It's called middle lane hogging, and it's actually illegal!
Otherwise, don't feel pressured to go faster than you're comfortable with, and check your blind spot when changing lanes. And indicate!
Where's @Dogamoto when you need him. Didn't he used to be a Driving Instructor?