Some points I learned when objecting to a planning application locally:
1. Not all applications go to committee. Some are OK'd by planning officers themselves. You want the application to go to committee, so find out from the local authority what the criteria are. It may be (eg) number of objections, of objections from local organisations, or ....
2. Objections are counted per letter (or maybe per address they come from) not per signatory. It doesn't matter if everyone sends in identical letters. So provide your neighbours with pro forma letters instead of asking them to add their signatures to yours.
3. Get the parish council (or similar) onside. They are usually automatically asked about applications and automatically don't object. If you give them reasons to object they may well be happy to do so.
4. Get your local councillor onside. He may be able to request that the application goes to committee. Also involve any councillors that may have a particular interest, eg one may have a special concern about tree preservation. You can usually get information about councillors from your local authority website.
5. Make sure that your grounds for objection are 'permitted grounds'. A lot of councils publish details of acceptable grounds for objection - they are all the same as they are set down by law. One set is at
http://www.richmond.gov.uk/what_is_a_valid_objection_to_a_planning_applicatio n.
6. Depersonalise your letter as far as possible, and list and number your points so that they have to be considered separately. Friends of the Earth have some excellent advice and a draft letter at
http://www.foe.co.uk/resource/how_tos/cyw_55_planning_applications.pdf.
7.At the end of the day the councillors will weigh up everyone's interests and do what seems to them to be right. This may go against all the recommendations from specialist departments such as traffic, environment etc if the councillors nevertheless believe that that's what they should do. If this happens you are pretty much stuck with it. You could go for judicial review, but it's costly, and even if you 'win' at a review, the courts only have the power to tell the council to reconsider. They can't tell them to reach a different decision. So in practice you have just one shot at objecting - it's worth trying your best to make it effective.
Good luck!