Always begin with a clean and sanitized lash extension area by making sure you use a high quality primer, which will help to create a good bond between the lash extension and the natural lash. (a sneaky tip from one lash artist to another is to use a little alcohol wipe on the lashes as an alternative to a primer. It will strip the lashes of all natural oils creating a nice solid bond. Just make sure you're allowing it enough time to evaporate on the eyelid and lashes as it can make them water)
Make sure you're using the correct size and length of lash extension for your clients natural lashes. It's never a one size fits all and longer is not always better. Using too long of a lash extension can cause the natural lash to droop and break, which will reduce retention and leave you with an unhappy client.
Invest in quality lash extension adhesives and replace them regularly. Of course we highly recommend our Onyx Adhesive because it's the bomb dot com.
Educate your clients on how to properly clean their lash extensions and explain the importance of regular washing to avoid product and debris buildup. If your client comes in and you can see their lashes aren't in the best condition, always give them a lash bath and dry them off completely before going in with a primer.
And lastly, make sure your clients are aware that certain products and environments can break down the adhesive bond and that should they choose to continue using them if retention is poor they will need to come in for more frequent fills.