How to optimize experience gain?

Well this answer depends alot on specific situations, as well as team composition and exactly what hero/role you play. Overall to maximize xp gain and gold income you just have to make sure you are always doing something, as a carry this is quite easy to understand, since all you do is rotate from lane to lane to jungle farming away

For roamers/initiators/gankers

Your job is to look for fight, pick offs and if there are non, get some farm, for maximum efficiency team wide you want all the lanes possible filled with a hero or 2, as well as jungle/ancients being stacked and farmed

For supports

Its the most difficult, since you have the least priority with farm, so you are to try and leech xp off your farmers if you can. Do remember that not all heroes on a team need levels as much as others, if your a support thats under leveled, that's fine, in fact if you see pro matches alot of the time supports are usually under leveled. Of course a few small tips for keeping up with levels:

• Remember to stack camps before killing them if the time is approaching a minute mark.
• If you are a support, try stacking large creep camps and the ancients whenever you have time (and are safe), even if you are not a support, if you are near a camp near the stacking time, dont be afraid to stack it.
• TP scrolls, use them, love them, if a lane is un-occupied its wasted gold and xp, dont let that stuff go to waste unless you are needed for a fight/gank.
• Lastly, put pressure on the opposition, even if its as simple as pushing in a a lane or dewarding a point on the map, if you make them feel scared or force them to respond you can waste alot of their time, and will help cement a lead or help you crawl back into a game.
• Avoid wasting time on engagements that are unnecessary and are probably not going to turn out well, some times letting a tower fall is more useful than spending alot of time/money on defending it, if you arent able to take a convincing teamfight, then letting tier 1/2 towers fall may be necessary (obviously if you can defend it though, try).
• Use courier and avoid going back to base unnecessarily, similarly, grabbing at least 1 regen item on a hero is extremely useful for the early/mid game since it allows you to stay in lanes for much longer and soak up more xp. (i dont necessarily mean big regen items like heart or perseverance, but even small items like a sages mask, bottle or urn can be extremely useful).