Disclaimer: My job has never been primarily front-end. I just muddle through it when I need to. So this advice may not be the greatest.
There are a number of things you can try:
- Keep the design simple, functional, and minimalist.
- Get the client to clearly describe the design they want. Basically, let the client be the designer.
- Subcontract some design work
- Acquire a number of decent images, layouts, templates, and color schemes that you can modify as needed. Guide the client toward one of these “happy paths”.
- Read up about design and grow your skill in that area over time.
You might also want to think about back-end skills. As a freelancer you might have to wear more than one hat to complete a project.