Re-using old materials
When you're a professional writer, you also need to re-use your work to make "new" material. This is an essence of how to make more money out of your writing.
For example, if you have a document which has been declined by one magazine, make sure to submit it to the magazine's competitors. Often one piece that works in one magazine works just as well in their competitor's so make sure to check out their market too.
If you have had your manuscript declined you can also re-submit it to the same magazine a few months or a year later. If it is an excellent text, it's a great idea to submit it once again. Magazines changes editors rapidly and a new pair of eyes may retake a look at your work and give it a new chance.
Also, you can re-publish manuscripts that have already been published in a magazine. When you do this, you need to take off around 20%-50% of the original price. Let us say that you first sold your text for £100 - to round up the numbers - then you can ask for anything between £50-£80 for your old article. Best of all, you don't need to change a word.
However, you need to check your contract with your first magazine to see if it is okay to re-publish it with another magazine. If you don't have a deal, you should be in the clear to re-use your work, but if you feel insecure always double check with the editor.
I usually wait around 3-6-12 months until I re-submit an old article/short story, it all depends on how often the magazine publish a new magazine. If it's daily, I don't wait too long until re-submitting it while if it's a monthly magazine I may wait 6-12 months. Though, if the piece was previously declined, I send it to a different magazine straight away. Then it's no point in waiting around.
If you're working on the American market, you need to go through agents to re-submit old texts. As I haven't had anything published in the US, I am only focusing on the British market.
I hope this was useful to you and wish you the best of luck in your writing career!