Write per cent, not "percent". If using the % sign, don't put a space between it and the digit:

Of the 5000 people who answered the survey, 10% (or 10 per cent) were French.

58%, 90%, etc.

When calculating percentages, beware the "rose by/fell by X%" construction: an increase from 3% to 5% is a 2-percentage-point increase or a 2-point increase, not a 2% increase

Never use the "per thousand" symbol, ‰

0.001 not 1‰

For smaller decimals, house style decrees scientific notation:

1 × 10^-6 not 0.000 001

Except when comparing percentages directly:

The company's market share fell from 0.01% to 0.000 43% (not 4.3 × 10^-4% in this case – keeping the decimal format accentuates the comparison)