Sometimes by helping others, we help ourselves. Consider volunteering at an animal shelter to help other pets and/or making a donation to a local rescue in honor of your pet.
Contact the staff of your pet's favorite park to see if you can sponsor a bench or tree affixed with an acknowledgment plaque memorializing your pet.
Write a letter from your pet to you. What would he/she thank you for? What would be some fond memories for him/her? What signs of affection would he/she remember?
Plant a special flower, tree, or shrub in the yard. Consider making family and friends part of the dedication and planting. Favorite memories can be shared.
Create a bookshelf shrine. Include their collar, favorite toy, etc.
Write a letter to your pet. Include fond memories and their adorable traits.
Many people deal with feelings of guilt following the decision to let their pet go. Guilt prevents us from dealing with grief. This poem, "Choices", may help.