Never use the default "I'd like to add you to my professional network on linkedin".

Give some context and info, even very preliminary, as to why you’re asking to connect.

Try to personalize the invite message as much as possible. Leadoku allows you to include the person's first name in the message. This has proven to increase acceptance rate. But beside a person's name you should try to include information that you already know about the receivers. For example - if you searched for people with the title "Graphic Designer", located in New York - than try to use "Graphic Designer" and "New York" in your message.