Great thread, loving the ideas.
We have friends who had no money the first Christmas they were together so they cut up newspapers and magazines to make paper chains to decorate their flat. Every year since, they have made paper chains, now with the strips they buy and the children help. They hang them in the hall and as tacky as it sounds they look fabulous every year.
It's worth a look in Poundland, B&M, home bargains etc as there are some real finds to be had (in amongst the tat!) For example, i like small baubles with a string of battery lights in plain glass vases - v low cost from the afore mentioned stores and can be reused. Or mini baubles in cocktail glasses make lovely table decorations. Baubles hung on different lengths of ribbon from eg curtain rails can look fab.
Candles add atmosphere, You can put tea lights in pretty shaped jars, with Christmas ribbon or raffia tied around the neck in a bow.
Agree re natural foliage - if you don't have holly/ivy etc in your garden, ask around your friends or on your local FB page. They spread a lot so am sure people would be happy to share.
Pretty bowls of seasonal sweets and chocolates are decorative too.
Invest in one more expensive decoration each year and your collection will grow over time. TK Maxx / Homesense has been my richest source of these over the years.