First of all, are you in an outfit? If so, what kind(zergfit/normal players)?
Secondly, I am no expert, but there are basically two subtypes of point holds for me:
Dominant point holds - Where you have the population and/or force multipliers to maintain a strong dominant hold on a point.
Non-dominant point holds - Where you lack both numbers and force multipliers.
In case of dominant point holds, Justicia has a lovely 2-minute overview of them. You basically either spam MAXes/Engineers with some Medics thrown in or you flood the point with all classes. Such point holds occur quite frequently in fights with one point that have 24-48+ pop on each side and the meat of the method is to simply deny access with superior firepower and numbers and these holds are generally virtually unbreakable without MAX crashes.
In case of non-dominant point holds(defending/capturing a base in an underpop, solo or as a part of a small group/squad) things change quite a bit. First things first, detection is of utmost importance. An infi with motion spotters makes holding the point that much easier. Secondly, you generally can't afford to just sit there in a corner. The more people the easier it is to def, since one person has to cover less flanks/entrances, but in general:
Keep moving and going around corners. If you have detection, surprise solo players and groups of 1-3 people(depending on your aiming skills) by strafing out of a corner and headshotting them, then popping back in. If you DON'T have detection, it is still better to surprise than be surprised. Pop out of a corner and check if someone is there, be mentally prepared to fire. You have the surprise advantage, as people generally don't expect active defense.
If you don't have enough people for each to cover their own entrance and lack detection, move between the entrances. Don't just cover one only to be backstabbed from the other. Generally speaking, sitting in one corner(however good the spot is) and camping works either only once or against total newbies.
Flank. If you know you or your group can't win a straight fight, either go and aggressively flank(if you are solo) or have some people defend and others flank(if you are in a group. Even a 2-man team can defeat a squad if they play their cards right.
Be on the lookout for MAXes. They generally only fear Heavies, but surprise C4 can work wonders and an AV Mana turret can be a very nasty surprise. If you know you can't kill a MAX in one go, flank him and win the war of attrition.
All classes are VERY useful in their own manner. If you are solo, go for either Infi(detection and surprise attacks) or Heavy(1v1 dominance, anti-max, group survival). Medic can work very well if you are good, but MAXes (try to) ruin your day.
If you are in a group, try to have at least one Infiltrator(for detection). The rest is up to you, but generally speaking LA is not the most useful class during a standard point hold(he is an offensive class after all), but can work VERY well if you have good vantage points. Having medics and engies is good(and a good player can do just as well with them in combat as with Heavy), but you have to remember that medkits/health/ammo can be replenished by respawning on your beacon, while firepower can not. Rocket Launchers are very important both against maxes or as crowd control.