We've got four minimally invasive (as in retake your color palette from green and red), go-well-with-your-existing decor ways to add and maintain that warm, cheery feeling in your interiors all year long:
Keep up the low, twinkly lights Don't take down all the twinkly lights as soon as the holidays are over; use string lights in innovative ways to enjoy low, warm lighting all year long. As Sarah Dobbins rounded up in her post Keep the Glow Going: Twinkle Lights Year-Round, there are plenty of ways to enjoy the warmth and magic these lights provide without making it look too holiday festive.
Bring in reminders of home No not your current home, your childhood home! Isn't part of the holidays going home and being around funny kid photos of yourself (a particular favorite ever was spotted in an Austin house tour we did long ago.) Or perhaps you have fond memories of a silly vintage item or piece of artwork that always reminded you of home and the holidays. Ask the parents if you can borrow/have it for display in your place to infuse your place with a little kitschy, nostalgic charm. Think about arranging these new/old items in funky, eye-catching vignettes when you've acquired them.
Add or maintain some glitter and shimmer There's just something so shiny to the holidays. Whether it's light bouncing off of reflective decor or glimmering ornaments on a tree, include some more glittery and shiny stuff in a few rooms (partnered with twinkly lights even better!) to keep that fun twinkly feeling in your space all year round. Need some ideas for some glittering gold and more shiny gold?
Reminders of friends Don't just check your Facebook every hour to surround yourself with friends' smiling faces. One of the great things about the holidays is getting to spend time with family and friends; update your wall photo collages with recent fun friend photos, include letters/cards/post cards from friends in the decor (perhaps taking the place of any hanging holiday card displays you may have had) and employ your creative friends to make something for your home to include in your art collection. Surround yourself with a balanced display of warmth-inducing friend and family photos.
(Images: As credited in above links)