You should consider voting Green if they have a decent chance in your area, or if there is no pressing reason to vote for anyone else (e.g. because your area is an entirely safe seat and there are no other excellent candidates to reward).
They have the best policy direction (although the actual details are deeply unrealistic in places), but will probably fall apart like the Lib Dems if they get anywhere near a coalition, because they similarly have little negotiation experience and not much funding; however, this is unlikely to happen this election, and a vote for them signals to the other parties that adopting Green-like policies is a worthwhile direction for obtaining votes.
You should consider voting Lib Dem if your local candidate is excellent, if they stand a chance of winning, or if you dislike Green policy elements or your local Green candidate too much to vote for them.
The Lib Dems also have excellent policy on paper, and pushing the other parties in their direction by voting for them is no bad thing. They will be a useful brake on the bad tendencies of any other party in coalition (they take the edge off Conservative screw-the-poor doctrine and Labour authoritarianism, even if not as much as you might like). Which candidate is important - if you can support a 'good' LD then you can help with the internal party struggle.
You should consider voting Labour if your seat is borderline Conservative / Bad Lib Dem / UKIP and your vote could make a difference, or you have an excellent local candidate.
Labour is not doing very well at staying on the left and has always had a huge authoritarian streak which is pretty unpleasant, but they will probably kill fewer people than the Conservatives if they get to form the largest minority or majority in the upcoming government. Again, supporting good candidates helps shift the balance of power within the party, which is vitally important.
You might consider voting Conservative if your local candidate is truly excellent, if the other candidates in your area are terrible, or if you are in a borderline UKIP seat where the Conservatives are the only real opposition.
The Conservatives have been pretty terrible and look set to continue that way / get worse as they chase UKIP, but they are still better than UKIP and some individual Conservatives may well be worth supporting - and some terrible Labour or Lib Dem MPs may well be worth opposing.
You should never consider voting UKIP, but you might consider _campaigning_ for them if you are sufficiently cynical to pull it off and live in a marginal Conservative seat which doesn't have a risk of actually going UKIP.
UKIP are a terrible party who exist primarily to make the other parties more xenophobic and generally afraid-of-the-Other. There are many valid problems with the EU, but that does not excuse voting UKIP at a national election - most of the major parties have promised a referendum anyway. Even supporting good candidates (if you can find any) / punishing bad candidates isn't worth signalling in their policy direction (or, worse, actually helping your seat go UKIP). If you want to signal anti-establishment, vote Green instead.