The best tactical voting guide is here. Other guides are inferior, including the Hang-em guide that George Monbiot recommends, the Independent, Guardian and the Daily Mirror guides. I think they all have flaws.
The hang-em guide tells people to vote Lib Dem in some seats where the Labour candidate is clearly best placed to stop the Tory, the Mirror says vote Labour in 3 way marginals when the Lib Dem is best placed.
The one I've linked to is better because it advises a vote for Dr Taylor of Health Concern in Wyre Forest and Caroline Lucas of the Greens in Brighton Pavilion to stop the Tories and it also takes into account the Labour held seats where there is realistically no hope of stopping a Tory gain (using Ladbrokes as the best guide) and ignores them, concentrating on seats where tactical votes really could make a difference. The choice for progressives in these 'already lost' seats, (which sadly includes Hove and Brighton Kemptown) is which losing party do you want to enhance - Labour, Lib Dem or Green?
In these places we truly can vote with our hearts. We can also vote with our hearts in the 450 safe Labour/Tory seats because a vote here for the Lib Dem or Green will enhance their party next time. There are around 100 seats where a vote for the Lib Dem will maintain a Lib Dem MP or stop the Tory and around a 100 seats where a vote for Labour will stop the Tory.
Because of our undemocratic electoral system, we have to try and second guess what everybody else is going to do in our particular seat, if we want to have a chance of our vote counting. Even then, there is only a 150 or so seats out of 650 where we can make a real difference to what government we have.
Tactical voting is controversial because we all should be able to vote for the party we like the best, but only if we get PR can voting like that get quick results. To get PR quickly, we have to first affect the results now. Tactical voting is a shortcut to getting our way, but the drawback is it can slow progress for the party we really want to make a breakthrough. If people had always voted tactically would we ever have got Lib Dem and Green candidates in positions to win seats? I understand those who refuse to tactically vote, but at the same time, can we afford to wait decades in the hope of getting what we want.
In this election it is imperative we stop those who will entrench the present system. The Tories want fewer seats, not to save money as they claim, but because they want to win a majority on 35% of the vote, just like Labour did in 2005. This system will never be fair and the Tories will only entrench it. It only lets us 'throw out the government' as Cameron claims, if we choose between Labour and Conservative. Even then, it can still give victory to the THIRD placed party in the vote (as long as it is Labour or Tory). The Lib Dems could top the vote and still only get 15% of the seats, this is not democracy!
If we avoid a Tory majority we can at very worst at least keep the current boundaries where the Lib Dems can build on 90-100 seats and the Greens, Independents and Others can grow in strength (we could have 5-6 Greens in 'bohemian' urban areas, 1-2 UKIP in the home counties, 3-4 Respect in 'Muslim' seats and more regional independents like Dr Taylor in Wyre Forest).
A Tory majority will enlarge the boundaries to make it more difficult for smaller parties and independents to win seats and even harder for MPs to keep in touch with constituents views (In 1945 each MP represented 50,000, now it is 65,000, Tories will make it 80,000 constituents. The Tories only want larger seats because it benefits them). At best, if the Tories do worse than 300 seats, we can get PR, but only if Labour and the Lib Dems can win enough seats to form a coalition. This is why we need BOTH Labour and Lib Dems to do well and win seats to stop the Tories.
At present, the polls suggest the Tories will win around 300 seats, perhaps less than the 323 they need for a majority, but enough to stop Labour and the Lib Dems getting 323 seats between them to form a coalition. The only possible coalition at present will be Tory/Lib Dem and the Lib Dems should demand electoral reform. The Tories will reject electoral change out of hand and try to govern alone, with one eye on another quick election when it suits them. The Lib Dems will be challenged to vote down the Tory minority government and who knows how yet another election in one months time will play with the electorate. The Tories and their media friends have more than enough money to keep fighting elections and pumping out propaganda, the other parties do not. Like the banking collapse, the £25m donations the Tories have received from city bankers will cost the taxpayer billions to repay. Do we really want our supposed democracy in their hands?