What can supporters do mid game to help team?

Most support heroes (and especially Treant Protector/ Crystal Maiden) naturally fall off in the mid game, simply because that's how their kit is designed. These guys are incredibly strong early on, and your job is to secure the early game so your carries can gain an advantage.

While your impact later might drop off, dying and feeding is definitely a problem

Often, this comes down to positioning - you're most likely putting yourself in a vulnerable position where someone can kill you. Your aim should be to keep warding where vision is needed and to use your abilities as effectively you can in fights, without dying unnecessarily.

You know that you're an easy target later in the game, so try to constantly ask yourself how safe you are.

Pay attention to the map, and look for the enemy heroes. If there are heroes missing, then you probably shouldn't be pushing that lane solo! If you don't know where they are, then now is probably not the time to go and place wards by yourself.

Similarly, think about where you're standing before a fight

You probably shouldn't be the first one in, so make sure you stay back where you're not going to get caught out - that way you can still apply your disables after someone else initiates.

Think about what cheap items you could pick up to help save your life

A Force Staff for example can get you out of many difficult spots, as well as being all round useful for your team. A Ghost Scepter will make you temporarily immune to physical damage from the enemy carry, while still letting you get off your abilities. A Eul's Scepter of Divinity increases your move speed, and the active can be used to disable an enemy or make yourself temporarily invulnerable.

Some heroes naturally do not scale well into late-game, but there are items that can compensate.

Examples of great support items for all stages of the game for reference:

• Force Staff - This item can be cast on any non-magic immune target to push that unit forward in the direction the target is facing. This can be used defensively to save allies or yourself and offensively to push an enemy towards your team or a good initiator towards the enemy.
• Mekansm - This item when cast heals anyone in a radius around you and applies an armor buff to all units which helps the pushing power of your creeps.
• Pipe of Insight - When cast, this item applies a shield on all units within a certain radius that neglects 500 spell damage.
• Observer Ward & Smoke of Deceit - Wards for vision, Smoke of Deceit can be used to evade hostile wards to surprise your enemy.