Using chromosome finger puppets working my way through the various stages of mitosis/meiosis in the air helps.
1 finger = Chromosome
2 fingers (genetic material of a chromosome has duplicated in S phase) = sister chromatids of a single chromosome, joined at the knuckle (centromere.)
The number of centromeres present is equivalent to the number of chromosomes present (at least in my limited understanding.) Don't get confused just because there are two chromatids corresponding to the same chromosome joined together. 1 centromere joining the sister chromatids = 1 chromosome.
In prophase I of meiosis, there is crossing over of the nonsister chromatids of homologous chromosomes... using "chromosome finger puppets," hold up the index and middle fingers of both hands, and join the index fingers of both hands. It will make a w or h depending on how you look at it. H is for homologous chromosomes. The remaining middle fingers are flicking off the crossed non-sister chromatids for intermingling their genetic information. Those scurrilous fornicators - how dare they intermingle?
I dunno. I looked like an idiot doing the whole chromosomal finger puppet thing on my test, but it works for me.