When I want to get something done by my 8-year-old, I offer rewards for that activity, but still the rewards don't motivate him to do that task. The rewards are something that he loves the most, for example iPad time or the candies he loves.
Some of the tasks I ask him to do are like cleaning up his toys, reading the book he does not like etc.
What are the other ways I can get him motivated to get the task done? Is this the common behavior?
Rewards can actually be counterproductive. In psychology this is known as extrinsic versus intrinsic motivation. And yes, its very common. I well recall being forced to tidy my room and sit down to do my homework. Its an uphill struggle that every parent faces.
The trouble is that when you say "do this to get a reward" (extrinsic motivation) you immediately frame the issue in terms of the pain of doing what you want versus the pleasure of the reward. The pain is immediate, while the pleasure is remote. Thus the pain looms larger in the child's mind, and gets ever larger the more you insist on it. Also after a while the reward becomes the norm, so withdrawal of the reward starts to look more like a punishment.
Meanwhile any satisfaction that the child originally got out of doing the task (intrinsic motivation) is destroyed by the thought "I'm only doing this for the reward." Thus you have now made the problem worse rather than better.
The thing that is most likely to motivate a child is parental approval. Instead of saying "I'll give you a reward if you do this task", concentrate on getting them to do at least some of the task without a promise of a reward. Then give praise and encouragement for whatever positives you can find. E.g. "Well done. You've put a sock in the drawer. Can you put another one there too?".
Rewards should be used sparingly to emphasise a significant improvement. Like "Wow, you've read ten whole pages! That's more than you've ever done before. Lets have a cookie to celebrate.". Then you both have a cookie to emphasise the togetherness and approval part of it.
Edit: response to comments.
The distinction between reward and approval seems rather arbitrary at first sight. Isn't approval a form of reward?
In fact parental love and approval (they go together of course) plug into the psyche at a different level to material rewards. Consider the "monkey love" experiments of Harry Harlow. He gave baby monkeys a choice between a hard wire-frame "mother" which provided food and a soft furry "mother" which didn't provide food. The prevailing behaviourist (i.e. reward and punishment) thinking at the time predicted that the baby monkeys would become attached to the wire mother that provided food. In fact they became attached to the soft furry mother, only going to the wire mother when they were hungry. If they were frightened they immediately clung to the soft mother. This overturned a lot of thinking about human upbringing, replacing theories about reward and punishment with our current thinking which emphasises warmth and relationships as the emotional foundation on which we build our sense of self and our orientation to the world.
The trick in instilling intrinsic motivation in a child is to be the cloth mother, not the wire mother.
Children are very different in how their motivations work. I have two, one who is very externally motivated (i.e., what you describe above would work perfectly for him), and one who is more internally motivated.
For the child that is internally motivated, what works for me is finding the correct internal motivation, and finding the right way to fit it into his framework. My son for example is in a Montessori school, and so getting him to do chores (such as cleaning areas) works best when we fit it in the context of his school. They teach some of these skills at school, so we can give him "lessons" on the chore, and he then is in the mindset of working already.
We also focus on his internal motivating factors. He has several favorite shirts, for example, and a favorite plate, bowl, and cup. So when we need to fold laundry, it's quite easy to convince him: "Look, we have your favorite shirts in the laundry - let's fold them so they're ready for you to wear tomorrow!" When he needs to clean his room, again it's a matter of finding what motivates him - not a reward, but instead: "We're thinking of having a friend over, what do you think about cleaning your room so we can do that? After all, nobody wants a messy room when they have a friend over!"
These don't always work perfectly of course, in part because his motivations and desires simply don't align with ours sometimes; and he's only six, so, what can you do sometimes. But for the most part, we can get cooperation when we need it this way.
Our hope is that he ends up happier to do these chores when he's older because we're finding these internal motivations. His older brother is more externally motivated, and so it's both harder to find internal motivations with him, and, lazily, it's easier to get him to do things with simple external motivations. We have to fight ourselves sometimes to not solely use extrinsic motivations for the older child; it's just so easy! But he won't learn to want to do it himself, of course. For the intrinsically motivated child, it's actually easier, since saying "Hey, clean up your room so you can have some iPad time!" doesn't work - we find ourselves having to win his voluntary cooperation, which is what we'd prefer anyway.
This PsychCentral article, while not directly about motivations, goes into some details that I find useful, particularly a few of the different ways you can focus on the internal motivations - pride of place, for example, and establishing regular routines are both very helpful in this direction.
Another thing to consider, entirely separate from motivations, is how to remove barriers to cooperation. My older child (who is easily externally motivated) still has trouble cleaning a room, because it seems hard. So we help by removing the barriers that keep him from thinking he can do it. We show him how to do it; we show him how to break it up into smaller jobs; we are careful to give well defined, achievable goals. This article on Empowering Parents does a good job for example explaining some of this in detail.
