I'm relatively new at these, and until recently, I thought that some of the tougher puzzles required guessing to see if the guess resulted in a conflict.
I actually found one where a "Of A and B, one is this and the other is that" clue led to a side effect that was identical for both choices. I thought I had found the key. Not.
When you finally get to the clues of the type indicated above, they are really a very rich source of information.
Suppose you have "Of Mary and the person who arrived on Monday, one eats Wheaties for breakfast, and the other drives a Ford."
Normally you would start by X'ing out Mary/Monday, and Wheaties/Ford, and then checking both outcomes to see if only one of them can be true.
Sometimes, both outcomes are still possible. Here is what you can do to glean more logic from the clue.
First associate Mary with the same category as the entity she is paired with. In the example given above, that would be days of the week. If there are any days X'd out other than Monday, that's a good sign.
Suppose that Mary cannot be Tuesday or Wednesday. Then you know that the left side of the proposition can only be Monday, Thursday or Friday. Go check Wheaties and Ford to see if Tuesday and Wednesday are X'd out.
Second, associate Monday (still working on the left side of the proposition) with First Names (because that is the same category as Mary in the example). If Monday has any names X'd out other than Mary, that is good.
Third, associate Wheaties with cars and repeat the procedure above.
Finally, associate Ford with breakfast cereal and repeat again.
When even this doesn't yield enough constraints to finish the puzzle, do something similar for the "ordering" clues.
The clue is usually "A arrived earlier than B" or "A arrived the day before B arrived".
If A and B aren't in the same category, associate each one with the other's category to see if an X can be placed.
Finally, there are times where the puzzle is fairly complete, but things appear to be stuck. Look for cases where there are only two open cells in two columns and the two entities are related by a clue similar to "A arrived the day before B".
There will be one overlapping day between A and B, and you know that either A or B must occupy that day. So C, D, and E cannot be the day where A and B overlap. That might get you another X which will break things loose.
When I started using the above, I started to solve the tougher ones without guessing.
If you know of something easier yet, please post.
If this was helpful, I know of yet one more technique, and I'll post it if there is interest (and someone hasn't posted something a lot easier).