Post Protectors

• Many companies have recognized this problem and developed fence post protectors in a variety of styles, shapes and materials. These attach directly to the base of the post and prevent the string of the weed trimmer from making contact.
  
  

Keep Your Distance

• One obvious way to minimize damage to a fence post when trimming is to carefully avoid letting the string hit it. Angling the string or "blade" toward the ground will help as well, as you will likely hit dirt before hitting wood.
  
  

Keep the Wood Healthy

• Wood will deteriorate over time, especially when left exposed to the elements. Yearly applications of wood protectorate, stains or weather-proofing will keep the wood resistant to slips with the trimmer.
  
  

Consider Trimming by Hand

• Unless you have miles of Texas-style fencing, you may want to consider just trimming long grass and weeds around posts by hand. If will take longer, but it will alleviate any worry of fence post damage.