Reliability is key - turn up when you say you will, and work the hours you have agreed.
I am always at work when my cleaner comes and they have a key, so I'd always want good verifiable references and DBS.
Use initiative, unless asked not to - I haven't got the headspace or the common sense to keep on top of tasks that only need doing occasionally, and I love it when I realise my cleaner has cleaned all the lampshades or under the fridge, it makes me very happy! Check if there's anything fragile or that needs particular treatment or that they want you to leave alone.
Be thorough - I don't want someone to just hoover the middle of the carpet or wipe the kitchen surfaces without moving stuff, my DC can manage that!
Ask for feedback and act on it - I've only ever sacked 2 cleaners, because they did what they thought needed doing and didn't change what they did when asked. It doesn't matter if you think all shelves need dusting weekly, if the client prefers them done monthly and the time spent cleaning windows instead. Also say if you don't do something, e.g. working at height to clean cupboards.
Make sure you get the info you need at the start to do what the client wants, eg priorities, do they want you to tidy or only clean. I had a cleaner who used to arrange my DS's teddies in his cot, which was very sweet but I'd look at it and think 'you could have used that time to wipe the skirting board!'.
Let the client know in good time if they're running low on cleaning supplies, or you don't like a particular item they've provided or prefer something else - I dont mean demand they buy a £££ Hoover, but if they buy products that you don't think work best, or you want a particular hob cleaner or something, say so. If you break something or something isn't working right, let the client know.
Hold your own boundaries, good cleaners are hard to find - so don't let clients owe you money, or ask you to do tasks that aren't in your remit (unless you're happy to water plants or let the dog out or whatever, in which case charge accordingly). And know your worth - charge appropriately, and sack clients who misbehave in any way, if you're good there will be others wanting your services who will appreciate you!