Giving your boss an ultimatum should be a last resort. Try alternate routes, such as negotiation, before you make any sort of demand. If your boss feels as if you're holding him hostage, you might not get the results you want. However, if you've exhausted all your other options and feel you have no choice but to go with an ultimatum, you'll need to prepare beforehand and tread carefully to avoid negative consequences.
Consider All Angles
Since giving your boss an ultimatum could easily backfire, it's a move with some risk attached. For example, if you threaten to quit if you don't get a raise, your boss could let you go. Think about the possible outcomes if your ultimatum doesn't go your way, even if what you think you're presenting doesn't appear to have serious consequences. If you're demanding a promotion, for instance, are you leaving yourself open to a demotion? If you can't weather any potentially negative outcomes, you shouldn't give the ultimatum.
Practice what you'll say to your boss when you're giving your ultimatum and test your presentation out on other people. Ask your test audience for feedback on areas where you can improve and for a summary of what you're asking for once you're done. Depending on the feedback you get, you might have to tweak your presentation for clarity and to get the right tone. For instance, if you're told you sound angry, work on getting a more neutral tone. Don't try it on coworkers unless you can absolutely trust them. You don't want word getting back to your boss before you speak to him.
Back Yourself Up
Don't just leave it to your boss to rely on your words alone. You'll make a more convincing case if you have proof to back up your position. Insert evidence into your presentation. For example, if you're asking for a raise because your salary is well below those of other people in the same job in your market, cite the salary figures and where you got the information. Provide physical proof, such as a printout of the information you're citing, if you think your boss will question your facts.
Pick the Right Time
Don't spring an ultimatum on your boss at the wrong time, such as during a meeting or when he's focused on other things. Since it's a delicate situation, you need your boss' full attention and enough time to state your case. Schedule a meeting with your boss on a day when you know he'll have the time to hear you out and respond. You might want to consider his mood as well. If he was recently reprimanded, for example, wait to schedule the meeting, as he may already be on the defensive.
Give your boss more than one way to fulfill your demand if possible. For example, if you want a raise but would also consider a bonus for now, offer up both of these options to your boss. If you give your boss more than one choice, he's less likely to feel as if you're backing him into a corner with your request.