Luke,
To fill objects with diagonal stripes, by far the best way is to simply create a pattern with straight horizontal or vertical stripes, and then rotate the pattern within the object. This is much easier than getting diagonal lines to stitch correctly within the pattern design.
Any of the following methods can be used to rotate pattern fills within the selected objects:
1. Alt-click with the Rotate tool, or invoke Rotate from the Object>Transform menu, and in the resulting dialog, uncheck Objects and check Patterns, and enter your rotation angle.
2. In the Transform palette's menu, check "Transform Patterns Only", and then enter the desired rotation in that palette's angle field.
3. Hold down the tilde key (~) while rotating interactively with the Rotate tool. (Unfortunately there is no preview of the pattern contents during the drag. It just shows the bounding box rotating.)
The same methods can be used to Scale, Shear, Reflect or Move pattern fills (using the appropriate tool or dialog, of course.) You can save rotated (or otherwise transformed) pattern fills in the Graphic Styles palette.
If you decide that you still want to define a pattern design with 45° diagonal stripes, then the following method should be used:
1. Make a diagonal stroke or a diagonal rectangle that is long enough that the line including any stroke width
extends past the desired size of the tiling box.
2. Using the Move dialog, Copy-Move that diagonal line a specified distance horizontally or vertically, such as 12 pts.
Do not move it diagonally!
3. Ctrl-D to Transform Again until you get enough copies to make it look square.
4. Draw an unfilled, unstroked rectangle that is some multiple of the distance you moved in step 2, and small enough to be completely covered by your copied stripes. For example, if you moved your stripe 12 pts, you could make your tiling box 24 pts or 36 pts or 48 pts.
5. Send that rectangle to the back.
6. Select the rectangle and all the stripes and define your pattern.
There is an example file called "diagonal_stripes.ai" in my ftp download directory.
ftp://tpettit.best.vwh.net/
(I am posting the directory path rather than a direct link to the file, in case you want to browse the directory for other content.)