Personalised and handmade gifts always have that certain special touch, and homemade gifts can also help when there isn't much money to be spared. Gifts don't have to necessarily be material, why not treat your mum to a nice meal, or a day trip out. If you're the artistic type you could sew, paint or create something really unique for your Mum this year.
If all else fails, chocolates or flowers (or both!) tend to go down well, and if your Mum's a gardening fan then why not get a potted plant or even seeds as a twist on buying flowers, that way they'll last and will be a constant reminder of the gift you gave her.