As unorthodox as it sound, you need to start a campaign to get those 500 twitter followers that you want. It is said that ask and it shall be given, knock and the door shall be opened and seek and you shall find. So if you are looking to get that high number of twitter followers, it is time that you start asking for them.
Reach out to the people that you know, ask them to follow you. No, this s not a desperate move, No it does not make you look lonely and needy. Bear in mind your goal, you are the one who knows yourself and you know what you want. You need to get those 500 twitter followers for whatever reason thus you need to employ any and all methods possible to get those followers. Ask them to follow you. Play your part by making sure that your page is full of interesting and quality content that will keep your followers entertained and then start asking people to follow you. You can go as far as asking even people that you do not know; you need not confine yourself to certain boundaries.