Remembering past lives

For adults

For adults, they can not remember because it is very far over you can remember it.

Take it easy. Don't look too far, just in this life, you have forgoten everything in your 1-2 years old period already.

For babies

For babies, Take it easy. Just see the amnesia-patient, it is the same but the reborn is worse than amnesia because reborn-human has an amnesia which is caused by loosen brain, not just damage, also has an amnesia which is caused by psychological trauma, which is caused bad environment in mother's stomach, too. So the babies loose whole of their past life memories.

In very advance theory, abhidhamma, babies can not remember because all of their memory-organs [saññāya āyatana] have change to very weak and none ability, similar to the old paralysis-man's organs. Also, they have to live in stressful stomach that has 37 °C temperature for at least 7 months, without vision. This is called kammajja-vāta [moving/changing form that arise by kamma] in suttanta-pāli tradition.

Kammajja-vāta is one element of kammajja-rūpa [form that arise by kamma]. All organs arising at the beginning moment of life, especially human organs, are very weak, loose every ability, walk/see/listen/etc, from the past life.

Mind and it's factors, such as saññā (memory factor), arise depend on those kammajja-rūpa which is a elements of purejāta-paccaya (pre arising cause) in paṭṭhāna, the 7th canon of abhidhamma. If this paccaya, sense-organs, so weak, saññā that arise at these sense-organs also weak too. Then saññā cause of sati (mindfulness, recalling ability mind's factor) by the definition "sati thirasaññāpadaṭṭānā". So the babies loose whole of their past life memories.

Theravāda recalling skill practice

In theravāda tradition, you can recall it by jhāna-based ñāṇa, such as pupbenivāsānussati-ñāṇa or paccayapariggaha-ñāṇa. This tradition still going on at pa-auk monastery and some Burmese monastery.