Purge the electorate
Start with the registered voter lists. Find some way of purging the people most likely to support the wrong person. As an example, write letters to the illiterate and elderly telling them to reregister online to confirm their address or they'll be removed from the lists.
Gerrymandering
Moving the electoral boundaries to favour your outcome.
In the process of setting electoral districts, gerrymandering is a practice intended to establish a political advantage for a particular party or group by manipulating district boundaries. The resulting district is known as a gerrymander (/ˈdʒɛriˌmændər/); however, that word can also refer to the process. The term gerrymandering has negative connotations.
In addition to its use achieving desired electoral results for a particular party, gerrymandering may be used to help or hinder a particular demographic, such as a political, ethnic, racial, linguistic, religious, or class group, such as in U.S. federal voting district boundaries that produce a majority of constituents representative of African-American or other racial minorities, known as "majority-minority districts". Gerrymandering can also be used to protect incumbents. Wikipedia
Placement of polling stations
Now that you have very few poor, elderly, or vulnerable people on your electoral roll, you don't need to place polling stations where they can access them. Only placing them on the far side of town where all these people would have trouble accessing them will be fine.
At this point I've already pretty much won the election and I haven't needed to find a weakness in your polling system yet. Unfortunately all these methods are par for the course even in our "free" "modern" western democracies.
Gerrymandering is a favourite in the US, in the UK the Labour party has just run a massive soviet style purge of, ironically, the communists on their voting lists.