Have someone proofread the text. I once painted a glaring typo in 12 inch high letters on a 12 by 16 foot town entrance sign. Noticed it when the sign was finished. Oil based paint. Stayed up the entire night doing the fix.

Don't use more than two clearly readable fonts. Especially when using sans serif fonts (like Helvetica or Ariel) don't compress them to save space, as letters like I,T,L become harder to differentiate visually.

The smaller the poster, the simpler the graphics should be, but in any case, simple is usually better.

Try not to use colours that make your text hard to read - no black on red, purple on black, etc.