Having recently been applying for jobs (once successfully, hurrah!) it would have been well to have tackled this Thing a few months ago. But no matter: perhaps I can conjure up some useful advice from my recent experience?
Keep a list
Keeping a list, table, spreadsheet, notebook, or other record of everything notable you've done professionally both inside and outside work is absolutely invaluable when trying to write up a new CV or complete and application form. I keep mine divided up by the type of thing I did - attending a talk, speaking at an event, visiting a library, writing an article, etc. - but you could equally well use another organising principle. As long as the salient details are recorded (what, where, when) you can pick the things you need for each application. I also keep all my old applications and CVs, so I can pick (with care!) phrases from them, and so I can use previous lists of "tasks undertaken" in any given job as a basis for the new write-up.
I'm pretty ruthless and try to keep a note of everything I've done, even if it doesn't look particularly relevant to my current or future work. You never know when it might come in handy, and if I haven't recorded it I tend to forget that it happened at all. Seeing something on a list (e.g. a presentation about medical databases that I attended) will call to mind some of the things I thought and learnt at the time (e.g. that there's an awful lot of really good metatdata work going on in databases like pubmed), which might just be useful to bear in mind during the application process.
Make it relevant
Compared with, say, applying for a University place (and I went through the Cambridge application process, complete with moderately baffling interview questions such as "how would you explain the differences between the main two political parties to a Martian who'd just arrived on earth?"), applying for jobs always seems to me a bit like sitting an exam for which the answers are given in advance. I'm not saying it's easy, but most applications (certainly in the library world) come with a list of requirements that the candidate should meet. I find writing an application that meets each of these one-by-one--neatly supported with evidence from The List--pretty dull, really. The constant self-aggrandisement grows wearing, and there's no intellectual or creative challenge beyond trying not to start each sentence with "I". Still, I'm not complaining... I still have flashbacks to how I didn't mention attitudes to taxation in the Martian question (and how I wish I were joking).
Get some advice
As well as having a second pair of eyes look over your application for typos, style and tone, it can be invaluable to speak to colleagues, mentors or connections who know something about the place you're applying to. Obviously this isn't always possible, as you don't always want to advertise that you're applying for jobs in general, or a particular post in particular. However librarianship, especially in certain sectors, is quite a small world, and it's likely you know someone who knows a bit more about the ethos and aims of the place you're applying than you can glean from their website and the application materials.
This bit is hard, especially when you're applying for jobs because you're on a short-term contract or are currently out of work. But there's no point working somewhere that you're not happy: you'll only make yourself ill. So, at interview, make an effort to enjoy yourself and show your personality. If this doesn't go down well then you probably don't want to be at that place anyway...
To anyone reading this who's currently applying: good luck!