Hamid said:
Buddhists and Hinduists believe in rebirth. But as far as I know, they can't answer this question:
When in your future life you can't remember this life, it doesn't matter what the consequence of your acts are in your future life. you will be born totally a different person with no recall of the past. So why should you do something which you get its result in the next life?
Without recalling the past lives, the idea of rebirth is useless.
I think that there is a fundamental misunderstanding implicit in your question. Religions (or spiritual traditions) attempt to provide frameworks for existential questions, which allow you to conduct your life in a meaningful way.
I guess, if I'm reading what you're asking correctly, that you're saying that if you don't remember past lives, then they are meaningless, therefore the idea of reincarnation is meaningless as it doesn't offer any real guidance to people. You don't see the consequences of your actions, therefore you can do whatever you like, therefore there is no moral compass provided by reincarnation.
However, my take on this is that you need to step back a bit to see what the doctrine of reincarnation is intended to do.
Reincarnation is telling you what is fundamentally happening, albeit a step or two removed from your daily direct experience. In most religions, once you get past the surface layer of good/evil, heaven/hell, god/satan, whatever, you get to the insight that 'hell' is separation from God. Reincarnation is an explanation of how, and why, you are separated from God. It also offers a way of rejoining God.
The ultimate prize in reincarnation is not about creating a better next life, or even doing good deeds; it is to end the separation between 'you' and God. Once you are aware of this separation, then you realise the hollowness of pursuing material ends, whether they manifest in this life or the next; the rewards, the gains, are transitory and unreal.
By understanding this, and working toward the 'real' goal, you obtain spiritual contentment. In fact, once you understand the real goal, and believe in it, and work towards it, karmic effects and rewards become completely meaningless, as you are effectively operating outside of their bonds.