Another option is to make buoys.

Woooo, Unless you want to end up with several dots on your screen impossible to recognize, you need to remember to name waypoints and stuff because the remove Salvo tracker doesn't have a "location" but it tracks targets. I just did a quick and dirty setup to show you how hard it could be otherwise.

First just launched ready ordnance as you can see no target, then I added a waypoint (just general).

This is important especially when you have multiple search and destroy missions along with monitoring of Jump points and such. Reason? You may have to clean it up at some point, trust me. 

Oh, don't be worried, you can remove the waypoints as well, doing that doesn't cancel the target on the salvo screen.

Deep Space Tracking systems can have a lot of range, but they are quite a lot heavier than a bouy or a civilian station. You also have to place them on a body, which means it will orbit and get out of position. That makes them more situational. Not necessarily useless, however.

I only use these on purpose because
1) I RP that this is just the amount of stuff needed to do the FTL communication with HQ and fleets
and
2) I think it looks really funny when you have 10 different asteroids in a circle all of them with 1 or 2 DSTs to track 1 jump point

Has anybody tried tiny commercial space stations as sensor pickets?

Thinking about the problem of how to provide long-term sensor presence with low ongoing costs. Obviously these stations would have very limited capabilities since they are limited to commercial sensors, but they also should be very cheap and small, and you could probably carry them in a hangar to drop at points of interest.

Yes, many have done that.

Another option is to make bouys. A bouy is a missile with sensors, no warhead, no engine, and no fuel. The reactor will keep the sensors going forever and missiles have no upkeep, so bouys are a cheap way to monitor small locations. Ships can drop the bouy out of a missile launcher, or you can put the bouy on top of a transfer stage. The former requires the ship to travel to the destination to be monitored, which is fine for jump points but can be contraindicated for planets. A bouy on a missile can travel to a planet on its own, and no real harm is done if something shoots it down.

When I use bouys I design my survey ships to carry a few on missiles and few to be dropped off at jump points.

Unless you're going for deep space locations DST installations are probably the regular way of handling this problem. However, a tiny picket station could be easier to transport.

Deep Space Tracking systems can have a lot of range, but they are quite a lot heavier than a bouy or a civilian station. You also have to place them on a body, which means it will orbit and get out of position. That makes them more situational. Not necessarily useless, however.

Only active sensors or any sensors?

Well, considering that missile interception usually happens head-on, having a missile turn around and try to hit again is kinda weird.
Semi-related question, can missiles switch targets on the fly?

How do I get the missile chance to hit higher? The latest update makes it kinda hard?

The individual chance-to-hit can be improved by increasing engine boost, devoting more space to engine overall, adding terminal guidance, including ECCM, using multiple warheads or adding retargeting capability. The overall number of hits can be increased by adding decoys, improving missile ECM and using laser warheads.

Because there wasn't any meaningful decision to it - you wanted an Excel sheet to find the sweetspot and then that was it until you hit the next engine level. Not using the sweetspot just meant playing with a handicap for no reason.

A better engine and allocate more mass to agility will help I guess?