Energizing relay using short pulse signal
In the schematic below, when S1 is made, a low-going spike will be generated when C1 differentiates the level change caused by S1 pulling the junction of R1 and C1 to ground. That spike will pull U1-2 low, trigger the timer, force U1-3 high, and energize the relay.

When U1 is triggered, U1-7 goes open collector and Ct will start charging through Rt until U1-6 senses that it has reached 2/3 Vcc. When that happens, U1-7 will go low, quickly discharging Ct, forcing U1-3 low, and de-energizing the relay.

Note in the plot below that even if the switch is held closed for a long time, when the trigger spike dies away U1-2 will be pulled up to 12V, inhibiting U1 from being triggered until it times out, S1 is released and then pressed again, starting a new cycle.



And, just in case you want to play with the circuit, here's the LTspice circuit list:

Version 4
SHEET 1 948 680
WIRE -1120 -80 -1168 -80
WIRE -1040 -80 -1120 -80
WIRE -992 -80 -1040 -80
WIRE -896 -80 -928 -80
WIRE -800 -80 -896 -80
WIRE -688 -80 -800 -80
WIRE -320 -80 -688 -80
WIRE -288 -80 -320 -80
WIRE -1040 -32 -1040 -80
WIRE -896 -32 -896 -80
WIRE -688 -32 -688 -80
WIRE -800 -16 -800 -80
WIRE -464 16 -576 16
WIRE -256 16 -400 16
WIRE -1040 112 -1040 48
WIRE -896 112 -896 48
WIRE -896 112 -1040 112
WIRE -800 112 -800 48
WIRE -800 112 -896 112
WIRE -576 112 -576 16
WIRE -576 112 -800 112
WIRE -544 112 -576 112
WIRE -256 112 -256 16
WIRE -256 112 -320 112
WIRE -688 176 -688 48
WIRE -544 176 -688 176
WIRE -288 176 -288 -80
WIRE -288 176 -320 176
WIRE -688 240 -688 176
WIRE -544 240 -688 240
WIRE -224 240 -320 240
WIRE -288 304 -320 304
WIRE -1040 368 -1040 112
WIRE -976 368 -1040 368
WIRE -832 368 -896 368
WIRE -288 368 -288 304
WIRE -288 368 -832 368
WIRE -1168 400 -1168 -80
WIRE -1040 400 -1040 368
WIRE -1216 416 -1296 416
WIRE -832 416 -832 368
WIRE -688 416 -688 240
WIRE -1296 432 -1296 416
WIRE -1296 544 -1296 512
WIRE -1216 544 -1216 464
WIRE -1216 544 -1296 544
WIRE -1168 544 -1168 480
WIRE -1168 544 -1216 544
WIRE -1040 544 -1040 480
WIRE -1040 544 -1168 544
WIRE -832 544 -832 480
WIRE -832 544 -1040 544
WIRE -688 544 -688 480
WIRE -688 544 -832 544
WIRE -256 544 -256 112
WIRE -256 544 -688 544
WIRE -1296 640 -1296 544
FLAG -1296 640 0
FLAG -224 240 OUT
FLAG -320 -80 TRIG
FLAG -1120 -80 SWITCH
SYMBOL Misc\\NE555 -432 208 M0
SYMATTR InstName U1
SYMBOL res -912 -48 R0
SYMATTR InstName R2
SYMATTR Value 10K
SYMBOL res -1056 -48 R0
SYMATTR InstName R1
SYMATTR Value 10K
SYMBOL cap -928 -96 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 100n
SYMBOL voltage -1040 384 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 12
SYMBOL res -704 -48 R0
WINDOW 0 40 37 Left 2
WINDOW 3 41 63 Left 2
SYMATTR InstName R3
SYMATTR Value 1meg
SYMBOL cap -704 416 R0
WINDOW 0 27 3 Left 2
WINDOW 3 45 34 Left 2
SYMATTR InstName C3
SYMATTR Value 100n
SYMBOL voltage -1296 416 R0
WINDOW 0 -53 5 Left 2
WINDOW 3 -242 110 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V4
SYMATTR Value PULSE(0 12 250m 1m 1m 1 0 1)
SYMBOL sw -1168 496 M180
WINDOW 0 32 15 Left 2
WINDOW 3 32 44 Left 2
SYMATTR InstName S1
SYMBOL res -880 352 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 100k
SYMBOL cap -848 416 R0
SYMATTR InstName C2
SYMATTR Value 10n
SYMBOL diode -816 -16 R0
WINDOW 0 40 31 Left 2
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL cap -400 0 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C4
SYMATTR Value 100n
TEXT -1280 576 Left 2 !.model SW SW(Ron=10m Roff=1G Vt=6 Vh=0)
TEXT -1280 608 Left 2 !.tran 2 uic
TEXT -672 48 Left 2 ;Rt
TEXT -672 480 Left 2 ;Ct
TEXT -520 440 Left 2 ;Tout = 1.1 Rt Ct