Another option: If your 3D part is square to the WCS, you can rotate your UCS about one of the axis' so the z axis is in the direction of the plane you're drawing in.
I always have the UCS icon "on" so I know where I am in drawing (not sure if you do or not, just a suggestion).
(see attached images;)
Using the UCS command, rotate the UCS about the y axis (90 degrees), draw your circle (and extrude or whatever you're doing next).
Similar options if you use the view cube, or preset 3D views:
simply select that face of the cube, (or the view name under the view command) and set UCS to "view".
HTH
I use the 3 point option suggested by "JONTHEPOPE" a lot too.