After many years of travel and experimentation - my method is as follows;
- Japan or Korea: Very easy to find even cheap hotels with coin operated laundry. All countries should be like this, but oh well.
- If staying in a managed apartment or airbnb, try and find out with a laundry
- Never waste time and money booking a hotel with laundry. It will be stupidly overpriced, and you are going to pass by so many good hotels because of a worthless feature. Also, it's usually found only in 4+ star hotels, meaning you're also overpaying just to overpay on the laundry as well
- For hotels;
A) Pack enough clothes to last roughly ~2 weeks without needing laundry
B) For longer trips, the hotels in cities you are staying at the longest, at approximately 1.5 week intervals, make sure to find the hotel within a short walking distance of a self-service laundromat
Also doing things like staying with friends (who will have a washing machine), or using your m8s airbnb laundries if possible (when mixing and matching your travel with friends).
But overall, booking a hotel close (~500m) to a laundromat is the single best and easiest protip I have ever found, then you never have to worry about this at all. Just go in the morning to do some laundry, grab a coffee, then put it in the dryer, watch some videos, read the news, whatever for 20 minutes and it's all done.
I keep dirties in 2 laundry bags I bought off Amazon - best purchase ever, and then you can carry them around easily to the laundromat, on the way back just put them inside your backpack, can put it in a plastic bag if you need to for cleanliness (I always have a roll of large plastic bags with me during travel).