Here's a multi-pronged approach
diet:
ensure that you have healthy diet, full of protein, water, water-rich veggies and other foods that are rich in the nutrients needed for hair. Also ensure your system is clean so these nutrients aren't wasted. I've read anecdotes on this forum of people noticing that their new growth was thicker and shinier after upping their protein and water intake and cutting back on carbs and processed foods.
follicle/scalp stimulation: Wake up your follicles and invigorate them. stimulate your scalp manually with massages and increase circulation through exercise.
scalp care: make sure you have good "soil" for your hair to grow in. healthy, clean scalp = healthy hair (I think most ladies on lhcf already have this on lock)
apply castor oil to the scalp and strands: it has a thickening effect on strands as they're growing in
henna: to give the appearance of thickening the strands that are already on your head - henna/lawsome deposits itself on the strands. I think cassia has the same effect, without the color.
moisture/protein balance and sealing
steaming: infusing the hair with water will help with moisture and can "plump" the strands. I think westndnbeauty reported that her hair felt thicker after doing a steam treatment.
gentle handling: keep the strands on your head by treating your hair like fine silk
protect the length of your hair, esp the ends: even if you manage to increase how thick your strands grow in, it's a constant battle to keep those cuticle layers preserved. If you take a single strand of hair from a lot of people, you'll observe that it thins out as you move away from the follicle towards the ends, the oldest part. This is also what can give people the appearance of thinning ends even if they don't have visibly split ends.
Here are some helpful threads:
L
ow Density Support Threads
Where have you seen improvements
Who has went from Fine to Thick hair..... I think there might a more recent thread with a similar topic, I'll try to find it
If you search keywords "thickened" "thickened strands", etc. maybe more threads will come up