One of the most important things a freelance writer can do is maintain good records. Unfortunately--based on the number of posts I've seen on various writing forums--many don't.
I regularly see posts from writers who can't find a copy of a contract and want to know if any other writers have sold to the same market and might know what rights they've signed away, posts from writers who don't when or to what magazines they've sent their manuscripts, posts from writers who haven't been paid for published work and don't know who to contact, and any number of other posts that make it painfully obvious that those writers need a good lesson in record-keeping.
I admit my method isn't perfect, but it's quite effective. I know each manuscript that's out to an editor, which editor/publication I sent it to, when it left my hands, and whether it was submitted via e-mail or snail mail. I know where every manuscript has been and when it came back. I know which were rejected and which were sold. I have copies of all of my acceptance letters and/or contracts and I have contributor copies (or copies I purchased myself) of nearly everything that I know has seen print.
And, perhaps more importantly, I can lay may hands on all of it within a few minutes.
I know other writers with fancy spreadsheets and databases who handle much of this electronically, but I'm still a paper-and-folder guy. After all, most contracts still arrive on paper and most work still gets published on paper. Even if I maintained an electronic database of some kind, I'd still need file folders for the paper.
So I use file folders and file drawers. Every finished manuscript gets a file folder (which contains a copy of the ms., related research materials and, ultimately, a contract and a printed copy of the published work) and each folder moves through a series of file drawers--one for finished/not submitted; one for submitted; one for accepted; one for paid for/not published; one for published/not paid for; one for published and paid for; etc.
In this way I spend my time writing and submitting, not looking for lost documents.
Writing is a business. Maintaining records is part of doing business and developing and maintaining a good record-keeping system is especially important for a beginning writer to develop. After a few hundred sales, the task may be too daunting...