There's one fairly general technique which I missed from my list of Sudoku-solving techniques. Since a problem composed entirely of 2-clauses (that's statements of the form 'a or b') can be solved in linear time, it makes sense to take the relaxed problem including only the 2-clauses and see if anything can be deduced from that. In the case of Sudoku there are two types of 2-clauses, those of the form 'x appears in one of the two remaining available spots in this row/column/subsquare' and 'x or y are the remaining possible digits in this cell'.
Whether large Sudoku problems have enough 2-clauses to make deductions based on them alone at all common is unclear, but it seems plausible. Since this technique most definitely is not subsumed by the two techniques I gave previously, it should be included for the sake of completeness, practical or no.
