I've seen a number of opinions on conductive gels to ensure the best conductivity between battery terminals and battery cables. I'm interested in knowing an expert's opinion on the matter. I'm particularly interested in products that work well in harsh marine environments.
We really need more info on the batteries and type of connection.
Will the connection be permanent? In that case soldering or welding are the best choices. If you're connecting to wire, you'll need good strain relief. Any repetitive motion at the solder joint / weld will cause failure eventually. In a vehicle the battery may have a tendency to jostle around. Make sure that this will not cause flexture of the wire to battery connection.
For permanent connections you can coat the resulting joint (among other things) with conformal coating, which will help inhibit corrosion.
For removable connections, corrosion of the contacts will be your biggest enemy. Use gold contacts if possible. There are solutions such as "deoxit" which will inhibit corrosion of the contacts, but that won't work as well or last as long as gold plating.
Keep the battery compartment as dry as possible obviously. Consider using a water proof box with a desiccant packet, or a heat source.
Chemicals like bleach will destroy contacts faster than anything in my experience, so be sure to keep them away from your connectors.