Eliminate Standing Water in the Lawn
Standing water in the lawn is the most important thing to eliminate in your backyard when battling mosquitoes.
If you have areas of your yard with no grass or plantings, over time they can naturally erode and develop into uneven terrain which allows for pooling water to sit over long period of time where mosquitoes thrive.
In order to eliminate excess water in these areas, you may need to have a drainage system installed to relocate that unwanted water to another part of your yard or off the property.
Another solution may be to simply backfill and grade the area with additional soil and then install grass or drainage rock to prevent standing water.