Ask customers to give you written feedback, i.e. by email.
Display the nice things that they say about you throughout your promotional mix.
Have a separate web page for testimonials.
Testimonials are a form of ‘social proof’.
Use them liberally within sales proposals.
If you sell to businesses, you will find that some client companies are not keen for you use their names.
If that is the case, you could attribute the testimonial to (something like): ‘Director from the construction industry’
They are more believable than an enterprise telling you how great it is.