Sources for puzzles
If you want to skip the puzzle creation there are several good puzzle collections available online (most of them concentrate on hard puzzles). Some examples:
http://people.csse.uwa.edu.au/gordon/sudokumin.php
Gordon Royle's puzzles with only 17 clues (see the link section). Most puzzles are not very hard.
http://www.setbb.com/sudoku/viewtopi...&mforum=sudoku
Wapati's hard puzzles (no chains needed).
http://www.sudoku.com/boards/viewtopic.php?t=6291
The Patterns Game Results: Contain puzzles with various degrees of difficuly that all have the same initial shape. The last figure on each puzzle line is the rating according to Sudoku Explainer.
http://www.setbb.com/sudoku/viewtopi...&mforum=sudoku
Tarek's "Pearly6000" (very hard).
http://www.sudokuvault.com/top50000.zip
Ruud's top50000 (most are really hard).
http://www.sudocue.net/top10000.sdm
A subset of the top50000 (contains only the hardest puzzles from the larger list).
http://magictour.free.fr/top1465
The top1465, another great list with really hard puzzles.
source:
http://hodoku.sourceforge.net/en/docs_cre.php