To stimulate memories for someone with dementia, first work out what items may hold special significance for that person. Did they play a sport? An instrument? Spend a lot of time in the kitchen? The garden? Then collect relevant items in one location - a memory box, so they're easy to pull out.
Have the person with dementia hold each item and encourage them to share what that object brings to mind. You can talk about how it feels and what memories they associate with it.
The possibilities for what you might place in a memory box are endless. Use your creativity to create a memory-stimulating collection of items customised specifically to the person with dementia.
The following slides may hopefully give you some ideas.