This is a good article concerning tile layouts and using grid lines. Photo's are below.
When measuring the cuts on a diagonal measure off "Tip to Tip" across the tile. Single cuts along a fairly square wall can be done several at a time. The harder cuts are in corners and there are two different kinds that I have pictures of in the album below. I used some small scraps of tile to demonstrate where you want to measure off from.
If you grid the floor and pop chalk lines just measure off of that, allowing for the grout spacing and the 1/4" space you should leave if hitting a fixed object like walls or baseboards.By far the most difficult cuts are having to measure and place tile underneath door jambs and casing when you are running a diagonal. They can be tricky at first. I would recommend laying the field tiles and "stop" 2 rows short of running underneath the door jamb. Then I usually mark off grid lines for those 2 rows until you are past the door. Measure off the grid lines and dry lay the tile in those 2 rows to get a good fit. Pick up the dry-layed tiles and install the 2nd row that runs under the door "first". Then set the 1st row. If you don't do it that way you may not be able to get those 2nd row tiles underneath the jambs. Adjust the spacing evenly on those 2 rows after you set them.