Tip 4: grabbing a new value for one variable across requests
Imagine the situation, you have this annoying value popping up on every response, which you need in every subsequent request. For example something like a VIEWSTATE, which is quite common and really annoying. If you lose it or pick up the wrong (one request older) version, your request is invalid and your session broken.
So, the simplistic way to grab that thing is on every request add something like the Regular Expression Extractor with a regExp in it grabbing the correct VIEWSTATE. This however ends up being a bit of a mucky business, because you end up with a lot of RegExp Extractors all over the place, resulting in a very complex JMX file.
Put the Regular Expression Extractor with all stuff in the right places right at the top of your Thread Group. This will ensure that the Extractor is triggered on each and every request sampler, and thus you always have the most up to date (and correct) version of your variable.