Use economics.
Everything has costs and benefits, and people (while not perfect Homo Economicus), often if not always choose to do what gives them the most "win" (benefit-cost). The tricky part her is those two are not universal, and each person has different costs and benefits figured out using their own personal utility function - often, not even consciously.
Thus, the generic answer is: either raise the cost of what you want to stop, or raise the benefit of opposite behavior.
This can be achieved either by changing explicit costs/benefits, OR, by changing their utility function calculation.
The problem is that, even within the confines of your specific (and great) question, the specific answer will be "it depends".
Let's look at both situations.
1. Actual personal hygiene - teeth.
Unless your bf is somehow deeply special, there's no big explicit cost to brushing teeth (as usual with IPS, you should ask, not assume - maybe he's one of the pretty rare people with a gum problem that makes brushing teeth extra painful. But let's assume he's normal). The main cost is to remember doing so.
So, the problem in this specific case is with the utility function - he simply doesn't assign nearly a high enough value to brushed teeth.
The way to approach this is communication - other answers generally addressed the exact communication ways to do so in detail - you can explain the benefits of clean teeth to you (makes it pleasant to kiss), or disadvantages to unbrushed teeth (makes it gross to kiss; plus long term will cost tons of dental spending if you two are serious enough to worry about common finances).
Frankly, if he's being obstinate, considering low cost to him, I would actually go for a Trade War (yay! Modern topic reference on IPS!). Tell him you won't kiss him anymore unless his teeth are clean, since it grosses you out. If he asks why you never told him before, honestly explain that you were unwilling to start a confrontation since you like him so much. I know very few males who would willingly accept a trade of less tooth brushing at the cost of less kissing.
This approach applies to nearly all items of true personal hygiene like this, that are generally low-cost to the person (taking showers, wearing smelly unlaundried clothes, brushing hair minimally).
Also, hopefully it doesn't need to be said, but be sure that you communicate gently and not stick your foot in your mouth. Maybe he brushes often but has a gum disease or some other reason he has bad breath that is largely outside his control. Not the most likely explanation, but worth discussing gently before going into "brush more" mode.
2. Superficial looks - old clothes.
OK, this one is definitely a different situation, with different approaches. Here, the problem is that this is truly two wholly different subjective utility functions.
To him, the value of being dressed up is minimal. To you (based on "2 hours to get ready") it's a big value.
As such, even if the cost to both of you is about the same (at least order of magnitude), you evaluate it as "must, so worth it" and he "meh, not even remotely worth it".
The problem here is that, unlike situation #1, where the cost is objectively minimal, AND utility function - when properly explained - is reasonably objective - here utility function is truly, honestly subjective.
He may very well have a good point - he likes you for being YOU, and your looks for being your looks. NOT because you have made yourself extra better looking by adding nicer clothes (and, frankly, he can be one of the Aspergers people like me who honestly automatically normalizes the looks to adjust for nicer clothes bonus).
Not only does he possibly see no value in you being dressed up, he may even be offended or disturbed by your approach - something that you may not have considered before.
If you care that he dresses nicely, you don't like his looks naturally, which means you have no future together, since couples don't see each other every day in their best look, but their worst).
To put it into modern HR lingo, you don't care about his authenticity.
If you care that much about nicer looks, you're likely to dump him for a fancy dressing GQ type man.
Now, neither of these may be true (or it may be but you don't realize that), but they are indeed, genuine worries - either conscious or subconscious. Without addressing them, your and his utility functions have no hope of being reconciled.
So, to discuss this and resolve it:
First, try to honestly evaluate why this is so important to you. Including the hard to palate but necessary self reflection of just why you care so much about yours and his looks. It may very well be that you dress up in order to make YOURSELF feel better, and this has nothing to do with him. Or it could be that you care so much about what others in society thing that you are ashamed of being seen with him. Or a bunch of other reasons. Be honest with yourself.
Second, once you know your own motivation, discuss it with him, and explain to him why this is important to you.
E.g. "Honey, I network with a bunch of high status people, and my career can be negatively impacted by being seen with someone dressed like a hobo". OK, that stings, but it's honest and gives him an objective reason to care.
Third, try to see it HIS way.
Maybe he finds clothes shopping excruciating (my SO does, so do I). Maybe he has a figure that is hellishly difficult to find good clothes for. Maybe he simply isn't aware that social networking requires an effort to look better (that is typical for Aspergers types). Maybe he was raised to be "authentic" by hippie parents and thinks dressing up is a lie.
The point is, try to understand the reason for his utility function being so low.
Fourth, finally, try to negotiate.
Other answers went into some detail here.
Maybe you offer lower his cost of dressing better (Help him pick clothes. Clothes shop for him. Give him money for clothes, ala Pretty Woman in reverse).
Maybe you try to let him understand why this has so much value to you (just remember, he may not share your views at best, and actively be hurt by them at worst - the very worst case scenario is, your views make you two genuinely incompatible as a couple long term).
Maybe you convince him to raise HIS utility function. Explain how your brother landed important job/contract because someone was impressed by how he dressed. Acknowledge that society sucks this way, but that is how the world works.
May be you raise his benefit by offering something in trade (you dress up for me, I play some useless game with you I would otherwise prefer not to play). Or hell, by complimenting how sexy he looks in a suit and tie (it worked on me :) - especially if you demonstrate that in a way a guy will understand, not just say it :)
May be you negotiate something in the middle. He dresses up for some specific special occasions, you dress down for "normal" ones.
