Spend extra time on consultations, especially with lashes. Then reinforce the consult expectations throughout.
Ive had some clients that cant make up their mind and will change it repeatedly throughout a service. Eg, this is one tan session:
Client: I want it light I like to be
Me: Mixing it up for her skin tone
Client: In tent ready to spray - It will be medium wont it? I like it medium
Me: No it will be light like you asked. Would you like me to change it to medium?
Client: yes
Me: All done
Client: Omg it WILL be dark wont it? I like to be really dark.
Me: To clarify, no. You said you wanted it light. Then I mixed it and you changed your mind to medium. I changed it for you. So it will be medium, as you requested and it will not be dark.
Client: Ohh.*leaves happy as larry and leaves me a nice review*
Sometimes..people are like that. Like do and say things on a whim. Some complain about nothing. Literally nothing. This nail here? Its a bit erm. Hm. Oh yes very hmm what would you like me to do about it?
Refer to what they ask for over and over.
At the end, whilst handing them the mirror say: ‘So today you asked me for x lashes in x style, so I have used x as requested. You look great! How do you feel? Have I met your expectations today? Is there anything you feel I can improve on.
That is the kind of service that prevents complaints. But even then, theres always a weirdo here and there, or a competitor with a jealous grudge so just get on with it and take it in your stride.
Hope this helps!