Help your child practice non-letter sequencing tasks, like putting different blocks or coins in a particular order, and asking them to copy it. This helps them to focus their attention and observe details.
Have your child look at printed letters frequently. When you read aloud, follow the text with your finger to show your child the left-to-right orientation. Make sure they see books with just a few words to the page and large type, so that the letters are easy to distinguish.
Put a sticker at the top left-hand side of a page that they will use to practice writing. This reminds them where to start.
Use stencils or tracing to help with proper letter formation, and remind them of the correct orientation.
Work on particular letters until they improve. For instance, S and B are very frequently written backwards. If a particular letter is an issue for your child, try giving them a visual image to work with to help them remember which way the letter is formed--for instance, for an S, ask "which way is the snake looking?"