[latentflip]

The best tip I've been given for practice is to actually sit with a piece of paper and write out your thought process whilst solving these little problems.

It's feels a bit laborious at first, but I find it makes for much more effective practice than getting to the answer in a roundabout way, and not really knowing how you get there.

As in hackerblues' comment, sitting and explicitly writing: "If I write a 5 in this cell then the column will contain two 5s. Therefore, this cell can't contain a 5." is more effective than bumbling around and just getting it right, particularly when practicing with simple examples.