There is a theory that many people physically should resemble their past life self. While this might seem at first to be impossible in terms of changing race or sex between incarnations, there are many human features which vary in fairly similar ways across all races and sexes. For example. a high or low forehead, fine or coarse features, thick or thin eyebrows, high cheekbones, etc.
While this would be unlikely to be an absolute rule, there are interesting cases people have put on the web such as the case of General John B. Gordon/Jeffrey Keene
If people remember a past life as a foreigner, why can't they remember their past life language?
For an example, let's take the case of General George S. Patton who was fairly open about having what he strongly believed were past life memories. If Patton remembered his previous life as a Roman soldier, why wasn't his Latin better?
First of all, in his poem "Through a Glass Darkly", he openly admits that he had only fleeting memories of his past lives. He remembers being a Roman soldier and some flashes from that lifetime but he didn't remembered the lifetime with any great general clarity. In fact, he even admits that he can't remember exactly which battles he fought in many cases when he writes "I cannot name my battles, For the visions are not clear, Yet, I see the twisted faces / And I feel the rending spear." So he could recall some of the most memorable and often traumatic moments for himself as a human being, but many technical points - which can confuse many old soldiers - often eluded him. So I think perhaps we can forgive him for not being better at Latin.
However, under hypnosis this can be a much different matter. One famous example of someone who, when hypnotised, could speak a language to a level he had not achieved and with a dialect he could not have known, was of film actor Glenn Ford - who spoke 17th century French while regressed to a past life.
What factors decide where people are born in their next incarnation?
This is a common question for people to ask, and the answers can only be speculative at best. One common idea rules that a person in their next incarnation will be thrown together with the people they were most emotionally attached to - either by love (including friendship) or by hate. This would be a good incentive for people to never actually hate anyone they don't like, so that fate would hopefully not throw them together again. So a person may in some instances be reborn in a country where the people they are closest to have ended up.
We can also look at various examples for ideas. In one case, a man who claims to have been a German soldier in the First World War remembered in that lifetime seeing a poster advertising immigration to Canada and thinking that this could be a nice idea. He now lives in Canada, thereby giving some credence to the idea that one's own wishes may determine the place of rebirth.
How long are people supposed to spend between incarnations?
Looking at what people who remember a past life have to say, this would seem to be very variable. Some people claim to have been reborn in less than a year of their previous death, while others seem to take many years or even possibly decades to return. One reason for this in conventional mystical thinking is that a person who has passed on might want to spend many years as a "guide" or "guardian angel" for someone - perhaps a loved one. Also some lives are said to be so traumatic that a person's spirit might take a while to recuperate from it. Or the person might want to spend time with other loved ones in the astral realms before returning.
Why can't everyone remember their past lives?
If everyone could remember their past lives then there would be some serious problems. For example, if a person were murdered, they might go looking for revenge on their killer - or worse still, as everyone else would remember them from their past lives, they might find themselves being held to account for all their previous crimes/debts/etc. As well as this, people would be tormented by every unpleasant memory they had from countless previous lives. Forgetfulness even within a lifetime is very important therapeutically as it allows us to let go of the many painful situations which occur. As the saying goes: "Time heals all wounds". Forgetfulness between lives would allow us to start afresh as it were, without too much emotional baggage hanging over from the last visit. However with spiritual advancement would come the possibility of regaining access to those past lives, creating the challenge to deal with and understand the soul's path up to that point.
Does everyone just keep reincarnating forever?
If Paramanhansa Yoganada's book "Autobiography of a Yogi" is to be taken at face value, then the cycle of reincarnation can definitly be broken. His guru's guru, Lahiri Mahasaya, was said to have finally broken out of the cycle of rebirth by breaking free of the bonds of karma (see chapter 34). Perhaps this level of spiritual achievement is not even necessary to break the cycle for some, and possibly desire to exist somewhere else is simply enough. Of note, most religions which contain reincarnation usually see it as a limited cycle which doesn't go on forever - perhaps ending when the soul has fulfilled certain requirements.
Why does nobody I know seem to have past life memories?
It may seem odd that reincarnation is talked about and is a more commonly held idea than many people might think, but most people never hear first hand about anyone's insights into their past lives. While having a past life memory appears statistically not to happen to the majority of people, there are many people who do have such experiences. However these experiences are often seen as being intensely personal and too easily a cause for embarrassment or mockery from people who don't believe in such things. Hence the vast majority of people who have past life memories do not tend to share them with friends or coworkers. Having said that, once the subject crops up in conversation in an informal atmosphere it can be surprising what people will tell you if they are assured that you are open-minded and they are not going to be scoffed at.