I'm going to be talking specifically about hoyas in this post, because that's what I grow and know about. Not all of these factors will apply to all houseplants.
I'm going to attempt to list what I think helps my plants in order of most helpful to least helpful.
Most hoyas need at least 60% humidity in order to truly be happy. Many of them will grow in normal household conditions just fine, such as pothos and some philodendron.
But all hoyas will grow faster with higher humidity. In order to get high enough humidity in many locations you need to use something like a grow tent, greenhouse or an IKEA cabinet. I choose to use a Milsbo IKEA cabinet and you can read more about that setup here.
Learning about Vapor Pressure Deficit (VPD) has made a huge difference for me in growing happy, healthy plants. Before I learned about it the plants in my IKEA cabinets would sometimes be fine and sometimes grow really slow, get mold on the dirt, get foggy, or new leaves would turn brown and die.
After watching this video and reading this article to learn more about it, it all became so clear why I was having problems. The short explanation of VPD is that your humidity and temperature need to be balanced or things don't grow correctly.
What this meant for my cabinets is that the temperature was way too low in relation to the humidity. So now I heat my greenhouses year-round and match the heat in each cabinet to the humidity in each cabinet by following this chart.
What this means is once a day or so I look at the humidity on my hygrometer and, if necessary, adjust the thermostat for my heat mats so the temperature is correct in relation to my humidity. The details of how to do that are in my IKEA cabinet post.
The correct lighting is very important for your plants. If they don't have enough they will get leggy and grow slowly and if they have too much they will burn. My IKEA cabinet post has more details on which lights I use and why.
It's important to have a good quality, well draining soil with good nutrients in it. I really like my hoya soil mix and think it really helps my plants to grow faster.
It's important to fertilize your plants, especially as they use up the nutrients in the soil. I really like Dyna-Gro Foliage Pro since it has a good balance of nitrogen, potassium and phosphorus that help the foliage on houseplants grow nicely but it also has all 16 minerals essential for optimum plant growth.
I just use the recipe on the bottle, half it, and feed most times I water my plants. They seem to like this consistent, tiny amount of fertilizer and nutrients.
I know it seems a scary and drastic at first, but once you chop and propagate your plants, you will see first-hand that most houseplants love to be pruned every once in a while. It seems to wake them up and get them pushing out lots of new growth.
Plus, then you have baby plants for "insurance" purposes, to share with friends or to sell.
Type of Water
Houseplants don't love water straight from your tap - unless you are really lucky and don't have hard water and have a really good water source.
I happen to have a reverse osmosis water filter so I use that when I don't have rain water - but rain water is my first choice to water my indoor plants with.
It's naturally soft, has a good PH and your plants will love it.
We have rain barrels at the end of our drain spouts and I try to use that whenever there is some water in them.