When defending, you should shift to BOSTON (an acronym coined by famous bridge teacher/author Barbara Seagram) . That stands for "Bottom Of Something, Top Of Nothing." This is a way to tell your partner if you have interest in the suit you are shifting to. From, say, 
872, you would lead the 8. (Note that this is opposite from standard signalling--where throwing/playing an 8 would say you like the suit and playing/throwing a 2 would show dislike). From K73, you would lead the 3. A low-card shift means you are leading from something. When partner sees a high-card shift, he will assume it is top--from nothing.
You might also consider getting Larry's book on Defense HERE which includes a section on BOSTON or try Larry's Defense Webinar series which you can find HERE.
In the middle of the hand, you decide to shift to this suit. Which card should you (East) lead?
| DUMMY | |
1076   |
|
| EAST | |
| K52 |
2
5
K
In the middle of the hand, you decide to shift to this suit. Which card should you (West) lead?
| DUMMY | |
| J76 |
|
| WEST | |
| 9842 |
2
4
8
9
In the middle of the hand, you decide to shift to this suit. Which card should you (East) lead?
| DUMMY | |
| J43 |
|
| EAST | |
| AK72 |
2
7
K
A
In the middle of the hand, you decide to shift to this suit. Which card should you (West) lead?
| DUMMY | |
| 1064 |
|
| WEST | |
| Q972 |
2
7
9
Q
In the middle of the hand, you decide to shift to this suit. Which card should you (East) lead?
| DUMMY | |
| 1076 |
|
| EAST (you are playing after the dummy) |
|
| KJ98 |
8
9
J
K
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