A wonderful landscape design does not translate to a great landscape unless you hire quality Landscapers.  If you aren't a DIY-er then see the below list.
 I do design only, no installation.  I can recommend some contractors for your project if you would like.

Have they been a good communicator in meetings thus far?
Do they answer the phone?
Have you checked out their website?
Is their work rated anywhere online?
Are they on time to meetings?
Do you have a quote in writing with line items listed?
Are the plant sizes listed?
Did they cover the full scope of the project?
Are they a member of BBB (Better Business Bureau)?
Do they offer a reference list (and did you call those references) ?
Do they offer at least a ONE year warranty?
Are they going to be around in a year?
What is their change-order procedure?
Are you paying a deposit?  (do not pay in full up front)
Are they using Rainbird parts for irrigation?
Where do they buy their plants?
Is there someone on-site during the installation who knows English?
Can they read the landscape design and do they have questions?
Are they going to bring you samples of the mulch/ stone etc?