I often have students who find it really hard to roll their R's at the front of their mouth, especially when the R comes before a vowel. Here are two tips to help you get better at this:
Say 't' and then 'd' after each other really fast - td, td, td, td, td.
Why? When your tongue moves from saying 't' to 'd' really quickly you are actually moving your tongue in exactly the same way as when you say the Norwegian R. Repeat it a couple of times every day.
Put 'd' in front of the R.
So if the sentence is 'Rune ringte Roald', say: Drune dringte droald. The idea here is to get creative and make up sentences with several words starting with an R and then put a D before each word starting with R. And of course practice every day.
Why? When speaking the d-sound, you position you tongue in the correct place to make a rolling R, which makes it easier to pronounce the R. Do these tricks and very soon you'll be rolling your R's more easily ;)