Talk to one or both parents at meetings, or parent and teacher conferences. Tell them that parental participation is encouraged, and some students may need additional help to achieve success. Encourage an ongoing dialog with the parents, and give them your phone number so they know they can always contact you. Maintain a friendly attitude, so parents feel comfortable approaching you.
Send out a letter explaining upcoming projects and ask for direct parental participation.
Assign projects that require the parents to come to the school with their children to make a presentation. You can ask the kids to write a report about what their Mom or Dad does for a living, and then ask them to come to school to talk about it, and possibly bring tools or lightweight equipment that represents the kind of work they do.
Get excited every time you see or hear about a parent helping out. Tell the parent how much you appreciate the help, and this might make them more likely to do it again.
Send out a questionnaire to parents, asking them for feedback on what might make them more likely to help out with projects. Ask for feedback or advice on how to make the process easier for parents who work a lot.