How can I improve my baby's appetite? She's 8 months old and it seems that at some days, she eats a lot, but other days, she eats a little.

At your baby's age, it's still acceptable if your baby doesn't eat much because milk should be her primary food until 1 yo. As long as she's healthy and active, no need to force feed your child. It may do more harm than good. If you're really worried, you may consult your kid's pedia.