What is the best way to record a repeating a riff?

Do I just record it once and copy and paste as many time as required or should I play the whole thing through?

Also if I can copy and paste what should I do to ensure seamless loops?

i usually play the whole thing. i tried to copy and paste before and it seem to clip or something in between the two sound file.
Usually play the whole thing but if it's a looped sound you're after -

Record the riff once, and edit the bar so that it is just the riff, with no silence before or after. Do this by double clicking on the recorded audio, then editing it in the window that appears. Zoom in and ensure that the beginning and end of the loop cut at a place where the audio wave intersects the line, if that makes sense. This will avoid any clipping noise.

Then, when you have the file edited nicely, go back to the normal window and press Ctrl+K. This will bring up a dialogue box to ask you how many loops you wish. Type in the desired amount, and you are done.
There is poetry in despair.
Thanks for the help I usuall edit the audio track without using the editor, is there any advantage to using the editor? I have to zoom in a lot in the track view to make an accurate cut.

Anyway I think you can make loops seamless by setting the correct tempo in cubase and using the time warp tool, but I am not sure exactly how this works.
punch in & punch out feature should be in a DAW...so should automatic cross fading, but if not look into it