Just keep a notebook and pen under your pillow and write whatever you remember about your dreams as soon as you wake up.

legal commonly reported dream aids/potentiators:

Supplements:

Melatonin

5-HTP

Alpha-GPC (Choline)

GABA

Plants:

Mugwort - Artemisia vulgaris

Dream Herb - Calea zacatechichi

Blue Vervain - Verbena hastata (used by Pawnee Indians)

Wild Lettuce - Lactuca Virosa

Passionflower - Passiflora incarnata

Kava - Piper methysticum