You ask about haggling over money. Well, my strong advice is don't haggle in the early days. It's more important to get published than to get well paid. Just ask for a "normal fee" or "your going rate". Later, when the editor WANTS your continual contributions, start haggling. Find out what the top rate per 100 words is paid by that publication and ask for that. You won't get it, but it's a starting point. But please! Don't worry about haggling at this early stage. First, prove that you are a reliable and constant source of quality material, and you meet deadlines. This makes haggling MUCH easier.
The reason the timing matters so much comes down to one thing: bargaining power. In your first weeks you have almost none, because the editor can find another eager beginner tomorrow. After a year of reliable, on-time, clean copy, you have a great deal of it, because now you are hard to replace, and replacing you is a nuisance the editor would rather avoid. Haggling is simply the art of waiting until that scale has tipped in your favour.
When the time does come, a few habits help. Always talk money before you write, never after; once the work is done your bargaining power is gone. Let the editor name a figure first if you can, because you may be pleasantly surprised, and you can always ask for a little more but you can never claw back a number you pitched too low. Know the publication's top rate, as I said, and anchor near it.
And remember that money is not the only thing you can win. A better page, a regular slot, your expenses covered, a faster payment, all of these have value too. Above all, stay easy and friendly about it. The editor is not your enemy, just someone with a budget. Build the relationship first and the rate will follow, because editors pay their best money to the writers who make their lives easiest.