Just the first few duckduckgo results of a search for "usa in person voter fraud":
http://www.politifact.com/georgia/statements/2012/sep/19/naa...
http://dailysignal.com/2015/08/11/nearly-300-cases-the-exten...
http://politics.stackexchange.com/questions/2588/how-many-kn...
So, one source out of the first random 3 that duckduckgo gave me considers in-person voter impersonation to be a substantial problem. It cites a Heritage Foundation report containing "Nearly 300" cases. This is less than the 537 votes by which Bush defeated Gore in Florida, and spread across 50 states and more than 30 years, but it's possible that each of these instances could include many fraudulent votes. The report is linked, though, so we can just break it down. The instances citing impersonation are:
Shasta Nicole Crayton, Alabama, 2004, impersonated one person and was caught by one of the means I specifically listed
Eugene W. Victor, New Mexico, 2015, impersonated one person and turned himself in
Gary Copeland, Tennessee, 2011, impersonated two people and was caught by voter ID laws. Credit where it's due, there is some positive effect here.
Mary Lou Simpson, Tennessee, 2005, impersonated one person, unspecified how she was caught.
Hazel Brionne Woodward, Texas, 2015, got her son to impersonate her husband and was caught when the husband showed up to vote.
Reyna Almanza, Texas, 2011, got her son to impersonate his brother, who was incarcerated. Unspecified if she was caught just by someone noticing that or by other means.
Doris McFarland, Washington, 2005, cast her husband's absentee ballot. This appears to me to be mis-characterized absentee fraud, but I'll count it anyway.
Robert Victor Holmgren, Washington, 2005, "cast a ballot for his recently deceased wife". That seems unlikely to have been done in person, but I'll count it anyway.
That's nine votes across more than a decade and five states.
There are a number of instances of duplicate voting, which is where you vote as yourself in multiple jurisdictions, and registration fraud, where you register as multiple individuals. Voter ID laws plausibly prevent those, but it seems more sensible, efficient, and effective to take measures to protect the integrity of the voter registry instead of verifying identity at the polls. After all, if one wants to do this in a state with voter ID laws, all one needs to do is use absentee ballots.
You're right that absentee fraud is relatively effective (which is why all parties do it). It's just less cost effective before a certain point than making sure legitimate votes for you are cast, and legitimate votes for your opponent are not.
> Since nothing much has been done in the past 20 years, I don't expect anything to change when you inevitably find people on camera.
You can arrest those people and try them for election fraud, as has happened in the past.