Try not to use an individual word more than three times. You shouldn’t use any form of it more than three times, either. For example: “childrens” and “children’s” would be considered the same word by some engines because some engines go to the root of the word. It would be prudent to pick one of these words and stick with it on a particular page, then if you must account for the other word, use it on another page.
Use keyword phrases to the best of your ability.
Try to limit your character count to 1,000.
Use the most important keywords or keyword phrases first.
Should you separate your keywords with commas? There is evidence that if you use commas you are limiting your phrases to those within the commas. If you choose not to use commas, then you might open your page to more keyword phrases. For instance, if your tag is “eductional toys wooden boats” you might get ranked for “educational wooden boats” but if the tag reads “educational toys, wooden boats” you’d have less of a chance to get ranked for “educational wooden boats” because the comma separates the words. Personally, I use commas – it just helps me read the tags better.